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Feeding your fish properly is one of the two most important aspects of fishkeeping (the other being water quality). Properly fed fish are healthy, whereas fish offered the wrong foods may get sick, even die. Some fish need very specialised foods, while others are generalists. Particular foods can be crucial to getting fish into breeding condition, while other fish may refuse food for weeks after import, and need careful care if they are to survive.
There's a lot more to fish food than merely opening a can of flake.
About flake food
Before discussing the alternatives, it's worth asking an important question: is there anything wrong with flake food? You might think there is, since you'll read lots about live food and frozen food. But really, if your fish are taking flake food, then there's no reason not to use it. Flake food, from a good brand anyway, like Sera, Tetra, or Aquarian, is guaranteed to have the right balance of nutrients for most tropical fish. Some species should really have vegetarian rather than standard flake (guppies and mollies, for example), but beyond that, choose whatever flake food your fish prefer.
There are a few rules to flake food, though. The first is to remember it doesn't last forever. Once opened, a carton of the stuff is good for about three months. After than, some of the nutrients oxidise, and even if your fish eat it, they will be denied some of the key nutrition. In other words, buy tubs of flake small enough to be used up within a few months. While bigger tubs might be better value on paper, you won't be doing your fish any favours.
Another key rule is to remove uneaten flake as quickly as possible. Flake food is quite concentrated, and fish need only a little. A mouth-sized crumb is about equivalent to us eating a pork chop or a large potato. Most fish need only a little flake, perhaps two or three times a day, to do just fine.
Finally, try alternating brands. Fish can get bored of a single type of flake. You may eve choose to have two different flake foods on the go, and alternate them each day. A popular combination in this case is to use standard flake alongside some colour-enhancing flake. Colour-enhancing flake, incidentally, is flake with extra carotene and other organic compounds that naturally occur in things like shrimp. In the wild, these compounds make the feathers of flamingos turn pink, and in aquaria, they help keep the colours bright on many types of fish.
So why doesn't everyone just feed flake? The problem is that not all fish will accept, and even those that do, may need a supplement of some kind as well. Specialists like puffers and elephantnoses siimply won't take flake, while other species, like gobies and glassfish, may eat a bit, but not enough to live on. Do your research before buying that fish, and if it needs something other than flake, consider whether you are prepared to make the commitment to supply it with the right frozen or live foods as required.
Live and frozen food
The alternatives to flake are live and frozen foods. Live food is exactly what is says, living animals. Frozen foods come in two sorts, freeze dried, and wet frozen. Freeze dried foods are sold in tubs just like flake, and can be kept at room temperature. Wet frozen foods need to be stored in your freezer. Of the two, freeze dried foods are more convenient, but wet frozen foods tend to be more widely accepted by different species of fish. Try a bit of each, and see which your fish go for!
Carnivores
Most fish are predatory to some degree, either eating invertebrates, other fish, or both. Generally speaking, feeding predatory fish isn't difficult provided the fish will accept frozen or dead foods. Feeding species that require only live foods is a chore, and any aquarist considering such a species should consider the amount of time and effort required to secure a constant supply of prey before buying that fish.
Insects and small crustaceans -- The majority of aquarium fishes feed on aquatic insect larvae and other small invertebrates to a greater or less degree. Some are specialists, with particular modifications that help them catch things like mosquito larvae; the uspide-down catfish is one such example, it's inverted posture helps it find insect larvae hidden among floating plants. Besides aquatic insects, this sort of fish will also take small crustaceans, such as Daphnia (water fleas) and Artemia (brine shrimp).
Feeding these fish is usually easy. Frozen Daphnia and bloodworms make an excellent staple diet for these fish and most will accept such foods readily. One peculiarity I've noticed is that there is variation among brands, and sometimes my fish refuse bloodworms from one brand but gooble up another brand. It's a good idea to try out one pack of a frozen food just to see your fish like it before buying in bulk. Why buy in bulk at all? Most stores offer a discount, typically along the lines of buy 4 packs, get 1 free.
You can also obtain live Daphnia, brine shrimps, etc., relatively easily. It's nice to set up a half-barrel in your garden and fill it with a few plants, some sand, and water. You can seed the barrel with Daphnia from another pond (or as live food from an aquarium store), and after a few weeks, you'll be able to use a small net to regularly harvest Daphnia all through the spring, summer, and autumn. Brine shrimps are easily raised in soda bottles filled with seawater and agitated with an air stone.
Large crustaceans -- Big crusteaceans, such as shrimps and crabs, will be eaten by certain fish, particularly puffers. This does depend on size though, and a pufferfish can easily be injured by a crab or crayfish too large to be dispatched easily.
The safest and most easily obtained live crustaceans in the UK are brackish water river shrimps. These will survive for a few days in freshwater, plenty long enough for freshwater puffers to eat them up. In the US, glass shrimps of various kinds, some brackish, some freshwater, are offered for sale. Frozen krill, mysids, and unshelled shrimps can all be used to feed puffers and other crustacean-eaters.
Worms -- Some of the bottom-dwelling fish enjoy worms of various kinds. Spiny eels and elephantnoses are two examples of these. Earthworms will be greedily eaten by these fishes, as will things like Tubifex, although live Tubifex have fallen out of favour over the years because they are believed to carry parasites and other nasties. Bloodworms, while really insects, are a safe alternative for many worm-eating fish.
Again, you have the option of using live or frozen foods. Frozen bloodworms and Tuibifex are easily obtained, but earthworms usually need to be collected from the garden. While most fish adore live earthworms, do make sure they are collected from a clean source (i.e., not from somewhere recently sprayed with an insecticide).
Snails -- A few fish feed heavily on snails. Loaches and pufferfish are the classic examples. While loaches don't actually need snails in their diet, there's a good argument that pufferfish do, because the shells help wear down their teeth, which can become over-long if not trimmed.
Bizarrely, there are no suppliers of frozen snails, so if your fish needs them, you'll have to raise them yourself. Baby apple and mystery snails are easy enough to produce, and make a good supplement for medium and large sized puffers. Pond snails can also be good, but as with Tubifex, there's a risk of parasite transmission if the snails comes from a pond or stream that has fishes in it. Malayan livebearer snails will be eaten readily by large puffers, but the shells are very tough, and some aquarists believe they may fracture the teeth of small puffers, doing more harm than good.
Smaller fish -- Most predatory fish will happily take smaller fish, given the chance. Even fish as benign as neon tetras will eat livebearer fry, and as you scale up the fish, so their range of potential targets will increase. Angelfish and polka-dot catfish (Pimelodus pictus) are two types of fish often kept in community aquaria but known to be effecient predators on small species such as neons, guppies, and danios. The golden rule is simply to avoid mixing fish of dramatically different sizes, unless you know that the combination will work. Plecs, for example, however large, ignore smaller fish.
Deliberately feeding smaller fish to bigger ones is a very divisive issue. The majority of aquarists appear to be against it, but a sizeable minority maintain that it is a worthwhile practise. Essentially the argument is this: predatory fish would feed on live fish in the wild, so feeding them live fish gives them a natural and healthy diet. The opponents maintain that the vast majority of predatory fish can be trained to take alternative food (though this may need some time) and such foods have a much lower risk of introducing parasites than inexpensive "feeder" guppies or goldfish. There's obviously an ethical argument, too, as to whether the practise is cruel, though one may well ask whether feeding a live cricket or earthworm to a predatory catfish or cichlid is actually any more cruel than feeding it a goldfish.
If you do want to use "feeder fish", then do at least source them from a clean supply, and ensure the feeder fish have been well-fed before hand with a vitamin-rich food so that the predator gets a healthy and balanced diet. Predatory fish invariably need to be fed only periodically, often only once or twice a week.
Herbivores and omnivores
Relatively few fish are exclusively herbivorous, but many feed primarily on green foods only taking meaty foods like insect larvae periodically. These vegetarian fishes include the plecs, some of the big characins and barbs, and a few of the cichlids, particularly things like Tilapia. Besides needing green foods in their diet, these fish are highly likely to damage your aquarium plants. Plecs tend to be the exception here, but some may nibble on particularly tender species nonetheless.
Many fishes kept by aquarists are omnivores, and will eat both meaty foods and plant foods. These include most of the barbs, gouramis, and larger characins, as well as many catfish and cichlids, particularly those from the African lakes. For these omnivorous fish, feed them frozen or a good quality flake food as a staple, but supplement their diet with a portion of algae or vegetables a couple of times a week.
Algae -- Plecs, mollies, and many other fish eat green algae, and will only do well if offered some as a regular part of their diet. There may be sufficient in the aquarium for these fish, but more often than not, you will need to supplement this with algae flake, algae wafers, or some other prepared food. Check out the range at your aquarium store. As a general rule, midwater fish like mollies and killifish prefer the floating flakes, while the bottom feeders will do best with sinking pellets and wafers.
Vegetables -- Pretty well anything green in your kitchen is worth trying out. Old standards include courgette (zucchini), cucumber, carrot, and frozen peas. Blanched lettuce also works well. Other aquarists have found things like potato, spinach, melon rind, and broccoli stems to work well. Basically experiment as much as you want; short of feeding them chilli peppers or toadstools, you're unlikely to do any harm!
You can use something like a lead weight to keep the food at the bottom of the tank, as well as specialised tools such as a "Screwcumber" or a lettuce clip (a bulldog clip with a rubber sucker). One thing many beginners fail to realise is that these fish like their veggies soft, and usually won't eat them until they've been in the tank for a day or so.
Wood -- Wood appears to be an important source of roughage for plecs. Bogwood should always be provided for them to nibble on. One genus of plec, Panaque, appears to actually digest the stuff, and it will not live long if kept in a wood-free aquarium.
Scavengers
Often overlooked, many beginners assume the scavengers -- thinks like Corydoras and kuhli loaches -- will simply make do with the leftovers. They won't; and it's important you give them their own food at least two or three times a week. Catfish pellets are widely sold and inexpensive, and in most cases make an excellent staple. In some cases, you may need to find a specific food item for a certain species. Talking catfish and "freshwater" flatfish, for example, enjoy bloodworms.
Conclusion
This only skims the surface of a complex and important topic. The important thing is this: just as you'd check what size aquarium a fish needs, or what water chemistry, so to should you investigate its diet. Simply expecting all the fish in the tropical fish store to accept flake is unwise, because many won't.
There's a lot more to fish food than merely opening a can of flake.
About flake food
Before discussing the alternatives, it's worth asking an important question: is there anything wrong with flake food? You might think there is, since you'll read lots about live food and frozen food. But really, if your fish are taking flake food, then there's no reason not to use it. Flake food, from a good brand anyway, like Sera, Tetra, or Aquarian, is guaranteed to have the right balance of nutrients for most tropical fish. Some species should really have vegetarian rather than standard flake (guppies and mollies, for example), but beyond that, choose whatever flake food your fish prefer.
There are a few rules to flake food, though. The first is to remember it doesn't last forever. Once opened, a carton of the stuff is good for about three months. After than, some of the nutrients oxidise, and even if your fish eat it, they will be denied some of the key nutrition. In other words, buy tubs of flake small enough to be used up within a few months. While bigger tubs might be better value on paper, you won't be doing your fish any favours.
Another key rule is to remove uneaten flake as quickly as possible. Flake food is quite concentrated, and fish need only a little. A mouth-sized crumb is about equivalent to us eating a pork chop or a large potato. Most fish need only a little flake, perhaps two or three times a day, to do just fine.
Finally, try alternating brands. Fish can get bored of a single type of flake. You may eve choose to have two different flake foods on the go, and alternate them each day. A popular combination in this case is to use standard flake alongside some colour-enhancing flake. Colour-enhancing flake, incidentally, is flake with extra carotene and other organic compounds that naturally occur in things like shrimp. In the wild, these compounds make the feathers of flamingos turn pink, and in aquaria, they help keep the colours bright on many types of fish.
So why doesn't everyone just feed flake? The problem is that not all fish will accept, and even those that do, may need a supplement of some kind as well. Specialists like puffers and elephantnoses siimply won't take flake, while other species, like gobies and glassfish, may eat a bit, but not enough to live on. Do your research before buying that fish, and if it needs something other than flake, consider whether you are prepared to make the commitment to supply it with the right frozen or live foods as required.
Live and frozen food
The alternatives to flake are live and frozen foods. Live food is exactly what is says, living animals. Frozen foods come in two sorts, freeze dried, and wet frozen. Freeze dried foods are sold in tubs just like flake, and can be kept at room temperature. Wet frozen foods need to be stored in your freezer. Of the two, freeze dried foods are more convenient, but wet frozen foods tend to be more widely accepted by different species of fish. Try a bit of each, and see which your fish go for!
Carnivores
Most fish are predatory to some degree, either eating invertebrates, other fish, or both. Generally speaking, feeding predatory fish isn't difficult provided the fish will accept frozen or dead foods. Feeding species that require only live foods is a chore, and any aquarist considering such a species should consider the amount of time and effort required to secure a constant supply of prey before buying that fish.
Insects and small crustaceans -- The majority of aquarium fishes feed on aquatic insect larvae and other small invertebrates to a greater or less degree. Some are specialists, with particular modifications that help them catch things like mosquito larvae; the uspide-down catfish is one such example, it's inverted posture helps it find insect larvae hidden among floating plants. Besides aquatic insects, this sort of fish will also take small crustaceans, such as Daphnia (water fleas) and Artemia (brine shrimp).
Feeding these fish is usually easy. Frozen Daphnia and bloodworms make an excellent staple diet for these fish and most will accept such foods readily. One peculiarity I've noticed is that there is variation among brands, and sometimes my fish refuse bloodworms from one brand but gooble up another brand. It's a good idea to try out one pack of a frozen food just to see your fish like it before buying in bulk. Why buy in bulk at all? Most stores offer a discount, typically along the lines of buy 4 packs, get 1 free.
You can also obtain live Daphnia, brine shrimps, etc., relatively easily. It's nice to set up a half-barrel in your garden and fill it with a few plants, some sand, and water. You can seed the barrel with Daphnia from another pond (or as live food from an aquarium store), and after a few weeks, you'll be able to use a small net to regularly harvest Daphnia all through the spring, summer, and autumn. Brine shrimps are easily raised in soda bottles filled with seawater and agitated with an air stone.
Large crustaceans -- Big crusteaceans, such as shrimps and crabs, will be eaten by certain fish, particularly puffers. This does depend on size though, and a pufferfish can easily be injured by a crab or crayfish too large to be dispatched easily.
The safest and most easily obtained live crustaceans in the UK are brackish water river shrimps. These will survive for a few days in freshwater, plenty long enough for freshwater puffers to eat them up. In the US, glass shrimps of various kinds, some brackish, some freshwater, are offered for sale. Frozen krill, mysids, and unshelled shrimps can all be used to feed puffers and other crustacean-eaters.
Worms -- Some of the bottom-dwelling fish enjoy worms of various kinds. Spiny eels and elephantnoses are two examples of these. Earthworms will be greedily eaten by these fishes, as will things like Tubifex, although live Tubifex have fallen out of favour over the years because they are believed to carry parasites and other nasties. Bloodworms, while really insects, are a safe alternative for many worm-eating fish.
Again, you have the option of using live or frozen foods. Frozen bloodworms and Tuibifex are easily obtained, but earthworms usually need to be collected from the garden. While most fish adore live earthworms, do make sure they are collected from a clean source (i.e., not from somewhere recently sprayed with an insecticide).
Snails -- A few fish feed heavily on snails. Loaches and pufferfish are the classic examples. While loaches don't actually need snails in their diet, there's a good argument that pufferfish do, because the shells help wear down their teeth, which can become over-long if not trimmed.
Bizarrely, there are no suppliers of frozen snails, so if your fish needs them, you'll have to raise them yourself. Baby apple and mystery snails are easy enough to produce, and make a good supplement for medium and large sized puffers. Pond snails can also be good, but as with Tubifex, there's a risk of parasite transmission if the snails comes from a pond or stream that has fishes in it. Malayan livebearer snails will be eaten readily by large puffers, but the shells are very tough, and some aquarists believe they may fracture the teeth of small puffers, doing more harm than good.
Smaller fish -- Most predatory fish will happily take smaller fish, given the chance. Even fish as benign as neon tetras will eat livebearer fry, and as you scale up the fish, so their range of potential targets will increase. Angelfish and polka-dot catfish (Pimelodus pictus) are two types of fish often kept in community aquaria but known to be effecient predators on small species such as neons, guppies, and danios. The golden rule is simply to avoid mixing fish of dramatically different sizes, unless you know that the combination will work. Plecs, for example, however large, ignore smaller fish.
Deliberately feeding smaller fish to bigger ones is a very divisive issue. The majority of aquarists appear to be against it, but a sizeable minority maintain that it is a worthwhile practise. Essentially the argument is this: predatory fish would feed on live fish in the wild, so feeding them live fish gives them a natural and healthy diet. The opponents maintain that the vast majority of predatory fish can be trained to take alternative food (though this may need some time) and such foods have a much lower risk of introducing parasites than inexpensive "feeder" guppies or goldfish. There's obviously an ethical argument, too, as to whether the practise is cruel, though one may well ask whether feeding a live cricket or earthworm to a predatory catfish or cichlid is actually any more cruel than feeding it a goldfish.
If you do want to use "feeder fish", then do at least source them from a clean supply, and ensure the feeder fish have been well-fed before hand with a vitamin-rich food so that the predator gets a healthy and balanced diet. Predatory fish invariably need to be fed only periodically, often only once or twice a week.
Herbivores and omnivores
Relatively few fish are exclusively herbivorous, but many feed primarily on green foods only taking meaty foods like insect larvae periodically. These vegetarian fishes include the plecs, some of the big characins and barbs, and a few of the cichlids, particularly things like Tilapia. Besides needing green foods in their diet, these fish are highly likely to damage your aquarium plants. Plecs tend to be the exception here, but some may nibble on particularly tender species nonetheless.
Many fishes kept by aquarists are omnivores, and will eat both meaty foods and plant foods. These include most of the barbs, gouramis, and larger characins, as well as many catfish and cichlids, particularly those from the African lakes. For these omnivorous fish, feed them frozen or a good quality flake food as a staple, but supplement their diet with a portion of algae or vegetables a couple of times a week.
Algae -- Plecs, mollies, and many other fish eat green algae, and will only do well if offered some as a regular part of their diet. There may be sufficient in the aquarium for these fish, but more often than not, you will need to supplement this with algae flake, algae wafers, or some other prepared food. Check out the range at your aquarium store. As a general rule, midwater fish like mollies and killifish prefer the floating flakes, while the bottom feeders will do best with sinking pellets and wafers.
Vegetables -- Pretty well anything green in your kitchen is worth trying out. Old standards include courgette (zucchini), cucumber, carrot, and frozen peas. Blanched lettuce also works well. Other aquarists have found things like potato, spinach, melon rind, and broccoli stems to work well. Basically experiment as much as you want; short of feeding them chilli peppers or toadstools, you're unlikely to do any harm!
You can use something like a lead weight to keep the food at the bottom of the tank, as well as specialised tools such as a "Screwcumber" or a lettuce clip (a bulldog clip with a rubber sucker). One thing many beginners fail to realise is that these fish like their veggies soft, and usually won't eat them until they've been in the tank for a day or so.
Wood -- Wood appears to be an important source of roughage for plecs. Bogwood should always be provided for them to nibble on. One genus of plec, Panaque, appears to actually digest the stuff, and it will not live long if kept in a wood-free aquarium.
Scavengers
Often overlooked, many beginners assume the scavengers -- thinks like Corydoras and kuhli loaches -- will simply make do with the leftovers. They won't; and it's important you give them their own food at least two or three times a week. Catfish pellets are widely sold and inexpensive, and in most cases make an excellent staple. In some cases, you may need to find a specific food item for a certain species. Talking catfish and "freshwater" flatfish, for example, enjoy bloodworms.
Conclusion
This only skims the surface of a complex and important topic. The important thing is this: just as you'd check what size aquarium a fish needs, or what water chemistry, so to should you investigate its diet. Simply expecting all the fish in the tropical fish store to accept flake is unwise, because many won't.
