What follows is a list of peaceful tropical species that only get to about 1":
Pygmy cories (Corydoras pygmeaus): There are several species of small cories but these are my favourite. They are also quite hardy and relatively common and, due to their small size, are an ideal cory for a small tank. Many people will keep larger cories in 10 gallons but this is not ideal due to their activity levels; with the pygmy species you don't need to worry about that. They should be kept in groups as, like other cories, they are shoalers. A group of 3-4 is adequate but the more the better. One thing of note is that, unlike your typical cory catfish, pygmies spend a lot of time in the lower middle layers rather than on the bottom.
Temperature range: 22-26 deg C/ 72-79 deg F
Endler's livebearers (Poecilia endleri or Poecilia sp. or Poecilia reticulata - which is actually the guppy - depending on where you look): These are a livebearer very closely related to the guppy and quite easy to get hold of. They are hardier than guppies and smaller. Males are very strikingly colored with vibrant markings and unusualy-shaped tails with sword extensions and the like. Females are duller and look very similar to female guppies. If you are keeping them in a 10 gallon, it's best to stick to males only. For one, there's little point keeping females if you aren't breeding, secondly they are often harder to find and, lastly, and most importantly, endler's breed like rabbits and keeping a mixed group is not a good idea in a small tank. You quickly get over-run with fry. If you do end up with a mixed group, they should be kept at a ratio of 2 or more females per male to minimize the stress on the females. If, however, you go for males only, the number is of little importance and you could keep around 10 in a 10 gallon as they get to just about 1". Larger groups look most impressive.
Temperature range: 20-28 deg C/ 68-82 deg F
These are males. Females look like slightly under-sized, drab, grey female guppies.
Mosquitofish/Least killifish (Heterandria formosa): Another very small livebearer. These are a particularly small species, hardly ever getting to 1". Females are slightly larger than males. In a 10 gallon, to avoid overcrowding due to fry, it is best to stick with a single-sex group of at least 4 individuals but they can be kept as trios (1 male, with at least 2 females) if the tank isn't too heavily planted as the adults and other tankmates will eat most of the fry. These are very hardy fish and peaceful. Do not confuse them with the 'Western mosquito fish' (Gambusia affinis) which can be aggressive and nippy and grows larger.
Temperature range: 20-26 deg C/ 68-79 deg F
Spotted rasboras (Rasbora/Boraras maculatus): There are several species of small rasbora but this is probably one of the more common ones. Like most fish in its genus it is a schooler and does best in groups of 6 or more. A group of at least 4 would be ok though due to limmited space. Like many of the well-known rasboras, it will school very tightly if placed in a tank where it feels even slightly threatened or if its line of sight is always disrupted. They are more likely to stick together when kept in large groups in larger tanks though. Regardless, no community is truly complete without its schooling fish and this is one of the few that will do well in a very small tank.
Temperature range: 24-26 deg C/75-79 deg F
There are many other small rasboras, which also do well in small tanks, such as Trigonostigma/Rasbora hengeli (which has several common names: Hengel's Rasbora, Glowlight Rasbora, Porkchop Rasbora, Slender Harlequin - demonstrating why you should use the scientific name for research ).
Temperature range: 23-28 deg C/73-82 deg F
Another small rasbora species - the mosquito rasbora, Rasbora/Boraras brigittae , which grow to just over 1".
Temperature range: 25-28 deg C/ 77-82 deg F
These are harlequin rasboras which grow to about 1.5" and so, as they are also schooling fish, are better suited to tanks of at least 15 gallons. However, they demonstrate the superficial similarities you might get between rasboras. It's essential to learn to identify the species you are after and be aware of its scientific name. Harlequins also happen to be, in my opinion anyway, the best schooling fish for a medium-sized community set-up so, while they're not ideal for very small tanks, they make wonderful additions for average-sized communities in place of smaller species like the ones mentioned here.
Myers/pygmy hatchetfish (Carnegiella myersi): Another schooler, this is one of the smallest hatchetfish reaching a size just under 1". Due to their unusual body shape, these fish make a very interesting addition to peaceful communities. Their only major drawback is that they strictly occupy the upper layers so won't like to be kept with other top-dwelling fish. They also have a tendency to jump out of the water so the hood should be at least 6cm above the surface of the water and tight-fitting. Though they are relatively hardy, they are prone to being bullied by anything remotely nippy so be careful with what tankmates you choose for them. Again, these need to be kept in groups. Idealy, there should be at least 5 of their own kind per tank.
Temperature range: 23-26 deg C/73-79 deg F
These pictures are not of C. myersi but of a similar species: C. strigata. These are often labelled as 'marble hatchet fish' and grow to 1.5". Being somewhat larger than Myers hatchets, they are better suited to a larger set-up but require very peaceful conditions as they are prone to being bullied. Also, they do better in longer tanks (at least 15-20 gallons) due to high activity levels.
Now to my favourite group, the gouramies. There are a surprising number of gouramies that will work and thrive in a 10 gallon but I've only listed the hardier and more common ones. Just so you know, bettas are also gouramies so all those betta species count here too (though I have not mentioned them in any more detail)
Sparkling gouramies (Trichopsis pumilus): These are adorable and, unlike many other common gouramies, will spend more time in the middle layers rather than right at the top. They appreciate a well-planted aquarium and do best when kept as trios of 1 male and 2 females. These endeering little critters only get to about 1" and will even 'croak' or 'sing' when breeding or deciding territories. There are a few other Trichopsis species out there including the common croaking gourami. Keep in mind that, though many stay small and are suitable, croakers are really too big for a 10 gallon community and many of the others aren't as hardy either.
Temperature range: 25-28 deg C/77-82 deg F
Honey gouramies (Colisa chuna or Colisa sota - depending on how up-to-date your sources are): These are probably one of the most common gouramies and are often mis-identified or mis-labelled. Don't confuse them with the closely related dwarf gourami (Colisa lalia) which gets to 2" and, though suitable for a 10 gallon, is not ideal and far less hardy. Male honeys develop a lovely blue-black coloration on their underside when in breeding condition, which contrasts startlingly with their bright golden-yellow body color. Females are duller with a horizontal stripe running along their body but they make up for their lack of color by having a delicate appearnce that I always think is beautiful. These gouramies also prefer a planted tank and like to have floating plants to hide in. They also do best when kept as a trio. They get to 1.5" so are the largest fish I've listed here but they are still peaceful and would do well with anything else in the list except, perhaps, the hatchets as honeys prefer to stay in the upper layers as well.
Temperature range: 22-28 deg C/ 72-82 deg F
Note that these pictures are of wild-color fish (IMO the natural honey color is also the most beautiful).
And here is a wild-color female with the distinctive horizontal stripe. Juveniles also display this coloration but males soon grow out of it. Note that the little fish in the background of this picture are very young Endler's livebearers!
This is of a red color morph. Take care not to buy dyed honeys or confuse these with the very similar sunset/red dwarf gourami (Colisa lalia), which grows larger.
These are dwarf gouramies (Colisa lalia). There are several color morphs of this fish and some of them can easily be confused with the honey gourami. Dwarf gouramies grow larger and do better in slightly larger tanks though single males can be kept quite happily in a 10 gallon with a few other small fish. They grow to 2" and can be very prone to disease. Again, the similarity of this species to the honey demonstrates the importance of knowing the scientific name of the species you are after and of being relatively confident when it comes to identifying it. Note that the top picture is of a male and duller female. The last is a female. The rest are males.
Some suggested combinations (taken from another post of mine):
1 male honey gourami, 6 spotted rasboras, 4 pygmy cories
3 sparkling gouramies, 5 myers hatchetfish, 4 endler's livebearers.
4 pygmy cories, 3 sparkling gouramies, 1 male honey gourami
AND so on... you get the idea?
Whichever of the above fish you might consider, remember that you can keep a maximum of around 10 individuals (give or take a couple) but you should also remember that some need to be kept in specific size groups (which I have mentioned in all these cases). A lot of them can also be relatively tricky to find - but they are worth it as you can actualy make a community out of them in a way you can't with larger, but also more common, species of fish. Most LFS will order fish in for you or you can order online. Do make sure you fishless cycle the tank beforehand (read the links in my signature), take the time to research each species BEFORE you buy any and take any LFS advice with a grain (or two) of salt.
Besides the tiny, peaceful, community fish listed above, there are slightly larger or more common fish (guppies, platies, neon tetras, otos etc) which also do well in smaller tanks. The purpose of this post, however, was to provide a short list of miniscule tropical species so please forgive me for not coming up with a comprehensive (and extremely long) list.
Just as an extra note, 10 gallon tanks need not be peaceful fish communities. Perhaps consider some shell-dwelling cichlids (Neolamprologus multifasciatus being ideal), a species tank of some of the more fragile gouramies (ie: licorice), a killifish breeding tank (Ps. annulatus was suggested by BigC) or a dwarf puffer tank. Even a heavily planted shrimp tank can look quite impressive (http://www.petshrimp.com has a list of interesting species). There's a multitude of options out there - you just need to do some research (try www.google.com ).
Neolamprologus multifasciatus - the smallest shell-dwelling cichlid
Thanks so much to everyone who's posted pics!
Special thanks to:
bloozoo2 (for the suggestion and the beautiful endler picture)
Fella (for the stunning multi pic)
SRC (for finding the time to post all those useful pics)
lljdma06 (for the wonderful photos - particularly of the honey gouramies)
Torrean (for the lovely dwarf gourami pics)
Xebadir (for the cute male red honey pic)
basia (for that enchanting honey photo)
kimmers318 (you posted some amazing sparkler pics but I didn't get to them in time)
valleyrich (for notes on hatchets)
Edited by sylvia, 24 February 2007 - 06:24 PM.