fish keep dying!!

Question
My boyfriend and I have a 20 gallon-long freshwater aquarium. We only set it up about 2 months ago, and all but 3 fish have died! First we had african cichlids- all died within a week. Then we had south american cichlids- which lasted a bit longer but then died as well. Then we had 4 angelfish- 1 is still alive, then we got 2 spotted catfish- which are still alive. Even a pleco that we got to keep the tank clean has died on us! The pH is at a perfect 7.0! We dont know what else to check, we're thinking it must be some kind of chemical imbalance- but we dont know what else to test besides pH. PLEASE HELP! we're afraid to buy any new fish! What should we do?? Thanks so much!

Answer
It sounds to me like your fish died while the tank went through the nitrogen cycle.  Also, they could have died from stress...I don't know of any african cichlids that can be kept in a 20 long tank...aside from the shell dwellers.  The spotted cats and plecos are also too large for a 20 gallon.  There are only a few of the South American cichlids that can be kept in a 20 long as well..

Test the ammonia, nitrite and nitrate.  If the Ammonia is above .25...do a 50% water change. If the nitrite is above .25 do a 50% water change. If the nitrate is above 5...do a 50% water change.