well, I can't answer specifically about aquariums because I don't have one...but if this was a pond, I'd tell you that you DO need to do water changes. Amquel will bind the ammonia and you do need it....but you also need to test for nitrites (which usually accompany ammonia) and do water changes to dilute the nitrites. High nitrites cause Brown Blood disease. Salt will protect against that....but some fish can't handle salt. It sounds like you stocked that tank pretty heavly right away, which is not a good thing. Anytime a new tank or pond is started up, the stocking levels need to be kept very low until the bio-filter has had a chance to cycle. Once it's cycled, the bio filter comverts ammonia to nitrites and then to harmless nitrates. Don't trust that store to do your water testing for you....go get a good test kit and do it yourself. Tetra Laborette is a good one, and is not that expensive either (less than $20).