I have been loosing a number of fish. All rainbow fish. Went to my pet shop and found i have ammonia in my tank. I have been doing water changes every day with no success. Other fish like my congo and black tetras, gouramis,angelfish and catfish are doing ok. So my question is why only my rainbow fish are dying? I am about ready to tear down my aquarium and start over. I bought my 29 gallon tank used. Is it possible it was previously contaminated? I have had aquariums since i was a little kid so this problem i have is very frustrating. Anyone with any thoughts ? Thanks.