Sorry, I didn't see your replies before now. Shrimp could be a possibility I suppose. Can't help but feel disappointed as I'd envisaged something beautiful and sparkly... Shallow, I know. Thanks for your suggestions though. Much appreciated.
Shrimps have a really low bioload (don't produce much waste) but they are very picky about nitrAte. You could give shrimp a try if you wanted, just keep an eye on the levels. I would be happy to keep shrimp in a 40 litre.
Hmm, thinking about it, I think a planted tank is probably the way to go. I haven't had time to fully investigate but they do look good and, although I'm sure I couldn't replicate them, those Amano ones are beautiful. I'm assuming they are slightly lower maintenance than having a real fish in such a small space.
Thanks for your help EauRouge. I'm so glad I discovered this topic as otherwise I think I could have made some horrible mistakes.
With a small tank it's always going to be extra work than a larger tank. A 45 litre probably wouldn't be much more than a 40 litre. It's a sliding scale rather than a cut-off point. In theory you could keep a betta in a 30 litre tank, they're small fish and they're not very active- but you'd be constantly monitoring the water, doing loads of testing and water changes and it would be impossible to keep up unless you were an anti-social geek with nothing better to do <cough>. Also you'd need to know a fair bit about water chemistry so you can spot the early signs of a cock-up. In a small tank, when things go wrong they go wrong really quickly.
A water change just involves removing water and replacing it with fresh water. It's a good idea to clean the gravel/sand at the same time and clean the filter sponges in water taken from the tank (never rinse them under the tap!). For a 40 litre tank it would only take about 20 mins to do so it would be easy enough to do twice a week.
Weeeeel, you could just about squeeze a male betta in there with a few conditions-
You might need to do a couple of water changes a week. You'll have to test the water more often If your tap water nitrAte is high then you'll probably have to use a nitrAte remover or use RO water. If things do go wrong, you'll need somewhere to move him to. Holidays might be tricky, will you have someone that can do water changes when you're away?
There's not a minimum gap for ventilation AFAIK but you'll need the space for maintenance.
If you don't want to chance it with a fish then you could just plant it up. You can get some really spectacular planted tanks. Google Takashi Amano and you'll see what I mean.
So, we have inherited a tank which is built into the wall, between the kitchen and front room (I think it may have been a serving hatch at one time). The tank is visible from both rooms. The previous owners weren't using it but I think the people before them had fish in it.
Anyway, I decided it would be fun to keep some fish in it and ds and I both got really excited. I've been reading up on what I need to do (correct equipment, fishless cycle etc). Stupidly though, I only just calculated the volume and it's only 40 litres. I can't keep any fish in it can I?
Is there anything else I can keep in it instead (something watery, I mean. Not a snake or anything like that.)??
The alternative is to try and get a slightly larger tank that fits in the space and would hopefully hold 45l of water so we could get at least one or two fish. The hole in the wall that the tank fits into is 63cm wide, 26.5cm deep and 34 cm high. The current tank is only 25cm high, so really shallow. Does there need to be a certain clearance above the lid for ventilation? I've no idea if it's even possible to get a larger one, as it doesn't seem to be a standard size.
Arggh. I should have kept my mouth shut before doing some proper research.