I have had a 55 gallon fresh water aquarium for about 6 months. Every once in a while fish go missing. As far as I understand, I have all peaceful fish and my pet store (along with some experts I've talked to) are clueless as to where they are going. The water always tests fine, tank is cleaned every 2 weeks...all looks good. So far, I've lost about 11 fish (clowns, tetras, etc.) I do a count every night and would notice if one was ill or dying. Where are they?