What kind of catfish for my 5-gallon tetra tank?

[Shirleycat]

I have a 5-gallon tank with one large blackskirt and a lemon. There was a medium-size serpae in there until three days ago when I noticed the blackskirt had nipped off half his tail, so I moved the serpae to a spare 2-gallon (he's all alone). There was also a small albino cat, but I thought the blackskirt was harassing him too, so yesterday I moved the cat in with the serpae. Today I found the cat dead. The 5-gallon tank was well-established, and no sign of sickness in any of the guys, except for the nipped fins of the serpae. I don't know for sure that the blackskirt was bothering the cat. Since he appeared fine the day before, I'm guessing there was something about the 2-gallon tank that did him in. (The serpae is doing fine.) So, given the aggressive nature of the blackskirt, is there a cat that would do better? If I keep the serpae in the other tank, what kind of cat do you recommend for him (I'd probably put another serpae in too).

[tauaru]

Seriously, I think there are very few fish who will live well in a 5 gllon tank, Betas are probably the best. The problem is the volume of water, in a large tank there is more room for error in water conditions. It is really hard to keep a small tank stable over the long run. Cats are probably going to end up dying in a 5 gal, some of the small cory cats might be OK though. I see that you mentioned three different species of tetra but only one of each. Tetras are scooling fish and need others of the same species to survive. General rule is at least three of any particular species but more is always better. I think you can begin to see the issues with that many fish in a 5 gallon tank (Ammonia, Ammonia, Ammonia). If you are unable to put together a larger set up, which is understandable, look for fish that are solitary by nature or stay very small. Emerald eye tetras are tiny and you'd probably be able to keep 5 or so in a 5 gallon. Always keep in mind that you've got a tiny tank, water changes twice a week.

[Shirleycat]

Thanks tauaru. I know we should have done some research before getting into the aquarium business (boyfriend set it up and I thought he knew what he was doing; I ended up taking care of it). I've been told by others that having three different kinds of schooling fish is a problem. The serpae that I isolated in the two-gallon was looking bad today, so I moved him back to the 5-gallon and watched carefully. I now think the blackskirt got a bad rap. I never saw him nip at the serpae, even though the serpae was moving pretty slow today. The serpae died this evening. I now think the problem was fin rot. I dosed the tank with karacyn. I'm just going to leave it for now, with just the blackskirt and lemon, and make sure they're okay before even thinking about adding anything else. I thought all fish swam in schools. Can you tell me what fish don't? And thanks too for the advice to make water changes twice a week. I've been doing it every two weeks (based on my research on the web). But I did a change today (a week after the last one). I will make more frequent water changes.

[tauaru]

It sounds like you are wiliing to take care of these little guys, which is awesome. Sometimes when people find out there's work involved with keeping fish they kind drift away. I would peck the guyd in your local fish stores brains, except in petco/smart (those corporations do not require any knowledge in order to work there), most private store workers are at least somewhat knowledgeable about what you can get for your tank.
You are right almost all fish school (or scoal, which is slightly different) in some way, there are however many non schooling fish, albeit they are usually larger predatory fish. Schooling is a protection method that sacrifices one or two to save the group. As far as fish for tanks that don't school...I don't think catfish school at all. Puffers don't school, many cichlids don't school...I'm sure there are others that I can't think of. Always ask somebody, hopefully they are able to tell you if they do or do not know (watch out, alot of people will not say 'I don't know' they'll make stuff up before admitting that)
I talked to some friends about a small size tank like yours and we all agreed; 8 or so emerald eye tetras and a few otocinclus or cory cats (smaller cory species) would be cool in there. You could probably do some BB puffers instead of the tetras. Just remember, it is a small tank so don't overpopulate, ammonia, nitrates, and nitrites will build up very quickly and they'll kill your fish.
Good luck!
P.S. I know it may seem cold but you might have to start the tank over completely with new fish, sorry