ronanphilip
Fish Crazy
what are your opinions on this stocking level article from thinkfish.co.uk
Stocking Levels this article falls under the subject: Knowledge (11)Like many subjects in the fish-keeping hobby, working out stocking levels can be difficult since there are so many conflicting sources of advice. In the case of stocking levels there are also many factors to consider which will vary the amount of fish you should stock, so any given formula can only act as a rough guide. To make things worse, the most common formula given for stocking levels is hopelessly outdated, yet still regularly seen advised to fish keepers.
The old formula
A standard formula seen for working out stocking levels for tropical freshwater fish is to allow 30cm of fish length per square foot of surface area. In a 30cm long and 30cm wide tank, this gives 30cm of fish. In a 120cm (four foot) by 45cm wide (1.5 feet) tank this gives us 180cm of fish. At first glance, 180cm of fish appears a reasonable number of fish – until we break it down and look at what that actually means we can have. Starting with a few small fish, say Neon Tetras, which normally reach 3-4cm, although we will go with 3cm, we could have 60 Neon's. This seems a reasonable amount, but you'd be surprised how sparse 60 Neon's look in a four foot tank - your local fish shop, for example, would normally have a few hundred Neon's in a two foot tank, but that will be discussed later on. In reality we would not just have Neon's, we would have a mix of fish, so based on our 180cm limit we could have the following in our 120x45cm tank:
15 Neon's (15 x 3cm) 45cmSo in our quite wide (45cm) 120cm long tank we can only get 33 small fish in which is not a lot. Looking at medium sized fish like Gouramies or Rainbowfish (10-15cm), we could only have 12 fish, and that's not including any catfish or scavengers. Another way to look at it is a 60cm (two foot) tank would be fully stocked with 12 Neon's (36cm) and four Corydoras (24cm) But if your local fish shop can have a few hundred fish in a two foot tank, why can we only have 16 small fish based on the 'standard' stocking formula?
6 Platies (6 x 6cm) 36cm
4 Black Mollies (4x8cm) 32cm
6 Corydoras (6 x 6cm) 36cm
2 Bristlenose algae eaters (2 x 15cm) 30cm
Total 33 fish 179cm
Why the old formula is wrong
The standard formula of 30cm of fish per square foot is an old formula, in fact, it is a very old formula. Whilst writing this article we looked in our book collection to see how far back the formula goes and found exactly the same formula in a book from 1980, and a similar recommendation in a very old book (given as a present) from 1964. The same book from 1964, when describing aquariums, has no mention of filtration and strongly recommends keeping a few plants, which happen to be particularly hardy and fast growing species. Because the stocking formula focuses on surface area rather than volume, this all indicates that wastes produced by the fish were used up by the plants rather than through filtration, and the factor which limited the number of fish was the surface area due to the volume of oxygen (and other gasses) which could pass through a still surface. The standard formula therefore is most likely based on an aquarium with no filtration, limited gas exchange, and hardy fish that could cope with fluctuations in water conditions.
Whilst there have been some variations to the standard formula, such as using volume rather than surface area, it remains basically the same, continually copied for probably over 50 years, yet an aquarium 50 years ago is very different to today's modern aquariums. We now have much better heating to give stability and filters to process waste products and keep oxygen levels high through surface water movement. The old formula is therefore completely inadequate for working out a decent guide for correct numbers of fish per tank, and we need a new solution
Working out a new formula
Assuming we have a sufficient amount of surface movement to introduce oxygen (fairly easy with modern pumps/filters) the only factor which has a significant bearing on how many fish we can keep healthy is the amount of waste product which can be processed by the filter. There is a proper formula for this, and it is used in the fish-farming industry. The formula is based on the total weight of fish, which gives a recommendation for the weight of food required, and the protein content of the food (which is what produces ammonia when processed) compared to the surface area of filtration media (which dictates how much bacteria can settle). Whilst we can obtain the surface area of our media quite easily from the manufacturers, it is another matter to start weighing our fish and measuring out our food by weight and protein content! As you can see, working out a formula for stocking levels is actually a very tricky thing to do, and we have not even started looking at the size of fish, swimming spaces, and compatibility!
Guestimation
Without a clear, accurate formula, working out stocking levels becomes a combination of educated guess work and common sense until someone comes up with something more robust. We know that stocking levels based on surface area are out of date and don't apply with today's equipment so we will stick to levels based on volume (normally you would buy a filter based on volume rather than tank dimensions or surface area)
If we start off with a formula of 1cm of fish per litre, we are quite close to the old style original formula, so we can assume this will work with tanks that have a minimal amount of filtration, such as an old style undergravel filter. Now lets assume we have replaced the undergravel filter with an oversized external filter, say one that can handle about 50% more volume than the aquarium. This type of filter will give us a massive amount of surface area for bacteria to process wastes, and a strong flow rate to oxygenate the water. We can confidently say that this type of filter will cope with several times more fish than the undergravel, but to be safe if we simply double the amount, we can keep 2cm of fish per litre.
We now have two formulas of 1cm of fish per litre with an undergravel filter, and 2cm of fish per litre with an oversized external filter. To get a rough idea of what we can keep in different tanks we simply create a scale of formulas between different types of filtration going from worst to best as follows:
Undergravel filter 1cm per litre
Internal filter 1.2cm per litre
Oversized internal filter 1.4cm per litre
External filter 1.7cm per litre
Oversized external filter 2cm per litre
Going back to our 120x45cm tank from the start, for which we had a stocking level of 180cm using the old formula, we can see what happens with the new formulas. A tank measuring 120x45x45cm has a capacity of 243 litres although we will take off 10 percent for décor etc. so around 220 litres. Based on the new formulas, depending on the filtration type, we can stock between 220cm and 440cm of fish. In most cases a tank this size will have an external filter, so around 375cm of fish is about right. This amount of fish works out at roughly double the amount given on the old formula, and allows a much more reasonable number of fish to be stocked. Remember though, that even this new method is just a guide and depending on the type of fish we keep, we can increase or decrease this guide level accordingly.
Bending the rules
Now we have a better guide to working out a rough stocking level, we can see how it works in real-life situations. Fish are of course, all different in shape, size, and behaviour so stocking levels need to be adjusted with a little common sense. In our 120cm tank with a stocking level of 375cm of fish, based purely on the stocking level we could keep 125 Neon's at about 3cm size or 8 or 9 fully grown 45cm Oscars. Keeping 125 Neon's in a four foot tank is no problem at all, in fact you could probably keep a shoal of 200 or so quite easily. Keeping 8 fully grown Oscars however is ridiculous, in fact even a pair of fully-grown Oscars would need a bigger tank. Small fish tend to eat proportionately less food, so any given total length, say 100cm of small fish, will require less food and produce less waste than 100cm of medium or large fish, so you can keep more small fish than the stocking level allows. Conversely, large fish produce more waste per cm of length (a 30cm fish will produce more waste than 10 x 3cm Neon's) and they also need a much larger swimming area, so you can keep less large fish than the stocking level allows. Stocking level guides will only therefore work for 'average' size fish, and you need to apply a little common sense to raise or lower your stocking level depending on the size of your fish.
Hard to believe?
Since the old method of stocking is so often repeated, it has become a 'standard rule' used by many fish keepers, even some experts still recommend the old guide without knowing they have probably overstocked their own tanks based on the old formula, or realizing how few fish the old rule allows for. The only way to prove that higher stocking levels are achievable is to find an established aquarium with plenty of fish, add up all the stock and see what it comes to.
As an example, the chart below shows the current stock of one of our tanks at Think Fish:
The aquarium holding the stock in the chart above holds 300 litres and is filtered by an external filter. Based on the new formula this tank has a stocking level guide of 510cm of fish (300 x 1.7) which is pretty close to the size of all its stock at fully grown size. In other words, using the old formula the tank would be heavily overstocked, but with the new formula it is near full capacity. This particular aquarium has been running for about two years and with this stock level for about a year. The fish are fed twice a day quite liberally with several different food types and we have had no health or water quality problems at all. In addition, the tank also holds about 30-40 shrimps, which we have not included in the stock list!
Stocking levels in the Think Fish Community Creator
Some of you may notice that the stocking level given in this sites Community Creator does not always match the fish sizes as given in the fishes profile. This is because the Community Creator uses a clever system of adjusting the total stocking length based on the type of fish you add. For instance, if you add large fish, it will use the full length, so a 30cm fish will normally take up 30cm of stocking space, and sometimes more. Smaller fish, like Neon's, although they may grow to 4cm, will only take up 2cm of stocking level in the Community Creator. This system accounts for individual differences in fish species; messy, big waste producing fish will take up more room, whilst small fish, which consume less and produce less waste, will take up less stocking capacity. Shrimps for example, require very little feeding, and most of the food they consume is algae or plant material, so they are recycling rather than increasing the waste load on the aquarium. In the Community Creator shrimps take up very little stocking level, so you can add more of them per tank than other fish. In the example above of one of our tanks you will notice that although the total length of fish is 503cm, the Community Creator gives us a total stock level of only 370cm. In the example tank, we have lots of small fish, like White Cloud Minnows, Danios, and Tetras, which produce little waste relative to their sizes. The Community Creator has taken this into account and given us more stocking room because the fish we have chosen produce little waste. This is why many people think the Community Creator's stocking levels are too high - in fact, rather than being too high, using the old stocking rule and not accounting for size gives a stock level which is too low.
Stocking levels in local fish shops
As mentioned earlier, it is not uncommon to see hundreds of small fish in a two foot tank at your local fish shop, and this level of stocking breaks even our new formulas – so how is it possible? The two main factors to consider here are the size of the fish and the filtration. Most fish on sale are young fish and are only a few cms in size, so they require less feed and produce less waste, allowing for a higher stocking level. In some shops, and nearly all newer shops, the tanks are connected on larger systems, so the volume of water is actually far greater than just the water in each tank, and the filtration for such large volumes is vastly better than anything we would use in a home aquarium, again allowing for much greater stocking levels. In theory, with enough filtration, you could keep a similar volume of fish at home, although most fish would prefer a bit more room.
Things to consider
Using the new formulas and a bit of adjustment (small fish = increase the stocking capacity, large fish = decrease the capacity) you can get a rough idea of how many fish you should have in your tank, but there are also other factors to consider. The size of tank a fish needs is a separate issue to stocking levels so it is always important to check a fish's requirements to make sure your tank is large enough for the fish when fully grown. Many fish also have temperaments that may require additional space to avoid conflicts, or in the case of Malawi cichlids, deliberate overcrowding combined with oversized filtration. Correct stocking levels also require correct feeding and maintenance regimes, overfeeding is essentially the same as overstocking and will cause water quality problems, and lack of maintenance will also cause problems. Therefore stocking level guides are always based on good feeding and maintenance, and require a little adjustment based on the type of fish you intend to keep
How much should we stock?
Whilst using newer formulas, and adjusting for larger or smaller fish may give us a much higher potential stock, it does not mean that we should stock higher. For instance, depending on the fish you choose, it is possible to overcrowd a tank using the Community Creator. This is because stocking levels are simply a guideline to how many fish can be kept in a certain volume or water with the right filtration, and the right feeding levels. Stocking level guidelines do not account for the space needed by fish to behave normally and live in a suitable environment. Unfortunately it is impossible to take this into account using formulas. For instance, you could have a tank containing a variety of bottom dwellers, nocturnal species, midwater swimmers and surface dwellers all with their own spaces and be near full capacity with little problems. Alternatively, you could have a large population of one type of fish occupying the same area at the same stock level, and be overstocked because the fish have not been given sufficient space. It is always best to understock than overstock, and stocking guidelines are only guidelines, so they need a little application of common sense.
Stocking levels in summary
The conclusion of all this is that to work out how many fish you can have is not at all straightforward. As discussed the old, or typical formulas are very outdated, yet still get passed around, so whilst you may see them more often, they are best avoided. The new formulas can be used as a guide, increasing the amount if you have shoals of small fish, and decreasing the amount if your fish are larger or produce more waste. We would tend to consider 8-15cm to be an 'average' sized fish, although again this varies on diet and habits of the species concerned. Alternatively, just use our Community Creator, which allows for these differences and see what level your tanks come out at – you may find you have a lot more (or less) room than you thought!
Key points:
Surface area is not an indicator of stocking level - Modern filters situated to create flow at the surface massively increase available surface area, the only situation where surface area becomes a problem is when an aquarium has a very high plant density (plants use up oxygen at night) or excessive waste build up (bacteria processing wastes use up more oxygen than fish)
Stocking level does not account for individual fishes needs - All fish need a minimum tank size based on the fishes size and temperament, this is never included in stocking levels which are simply a guide to the total length of fish you can keep in any given volume
Large fish produce more waste than the same length of small fish - A 30cm fish will have far more body mass, or weight, than 10 x 3cm fish, so it will require more feeding and produce more waste. Stocking levels need to be adjusted to take this into account.</B></I>
Stocking level guides only apply with good fish keeping practice - Stocking levels only work when tank maintenance, such as water changes, are carried out correctly, and when feeding is kept at the right amount.</B></I>
