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Your tank is overstocked, and it's not cycled. A 2.5 gallon tank is only big enough for about 2 fish. What you should probably do is keep all but 2 of your fish, and return the others back to the fish store. Definately take the algae eater back. What's probably happening is your
ammonia level has risen to a high level and your fish are starting to get sick. Tanks that are cycled do not have this problem, so you need to understand what a cycled tank is. Here is a good article to get you started:
http://www.fishlore.com/Articles/TheNitrogenCycle.htm
If you want to keep all your fish, I'd suggest you get a 20 gallon long tank for them ASAP. You will still be doing a lot of water changes until the tank is cycled, but at least you would have the right size tank. I'd still take the algae eater back though. Most of them you see in the pet stores are called common plecos, and they can grow to over a foot long. That fish will get way too big for even a 20 gal. tank. Later on as you get the hang of it, if you still want one, we can give you some suggestions and places to do research where you can find the right species for your tank.
