We need to know the tank's dimensions before though, I'm guessing the tank is around the 16" mark, remember that the volume will be before displacement.

Thats why I have just given the way to work it out because tropicals dont work on volume(unlike marines). Its just a rough guess that it would hold about 8 inches of fish. If it is a very shallow but long tank, it would hold more because of the increased surface area.

*Heidi*

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As you have suggested if the tank is long and narrow or very tall it will have a low surface area, putting restrictions on the size of the fish and how many you can stock. Larger fish will usually have a higher metabolic rate which means a greater demand for dissolved oxygen, this in turn means they need a larger surface area. You'll be surprised at the difference in oxygen demands between a fish that grows to an inch in length and a fish that can grow to twice that length.

I still do not think that it matters between an inch as long as it is not overstocked, that is what that calculation is for, and I belive that it is OATA that suggest that calculation, if they were talking about having 4-6 inch fish maybe!

*Heidi*

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The tank is very weirdly shaped - its curved at the front and is longer at the top of the tank than at the bottom. Thus it has quite a good surface area, but I can't really give dimensions with any real accuracy which is why I only provided the volume.
Its 15" high, varies from 14-16" in length, and varies from 8-9" in depth.

On the smallest of your measurements it gives you 11.2 inches of fish which if you work on 8-10 inches you wll be in comfortably. So anything like 8-10 one inch fish(neons etc) or 4-5 two inch fish(platies etc) obviously that is full grown size not the size you nessesarily buy them at

*Heidi*

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For me, its not just a case of how many inches of fish will fit into my tank. I want to make sure that the species I eventually decide upon will be happy in there; so that the tinyness of the tank doesn't stress them out, so that they have enough room to be as active as they need to be, as well as making sure that I don't overstock.
I'm sure that there are some 1" fish that would be happy in there, and there are probably other 1" fish that wouldn't. I'm trying to make sure that the requirements of the fish match up with the tank I have, which is why I asked for advice
My White Clouds only grow to, what? 1.5-2" in length? But after having seen how active they are in my big tank, I'd never put them in the 21 litre.

I don't know, I'm all confused now!

In which case I would suggest Siamese Fighers(1 Male and 1 or 2 Females or multipul females on their own) as they are slow moving and seem to prefer a smaller tank. Any of the little tetra's really, platies should again be no problem, nor should guppies. Would maybe not have danio's or barbs(especially tigers) as they are pretty boisterous. All of those fish would need a heater really.

*Heidi*

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While a siamese fighting fish would be suitable I would stick with just one, platies are an active species and do benefit from a spacious aquarium.

Ok, thank you to everyone who has replied!

I quite like the idea of a male Siamese fighter, or I've also had male Endlers suggested as a possibility.
I've bought a heater and I've only just today started to cycle the tank, so I've got a bit of time to mull over my options!