I set up my 25 gallon tank about a month ago, put in rocks from another tank, it has and undergravel filter, put in an airstone and left it for about a week, then I put in 3 tiger barbs and one pleco and they seemed fine after a week, so one day after work I decided to go and buy 4 more fish 2 green tiger barbs and 2 angel fish and when I came home to put them in the tank the pleco and one of the tiger barbs was dead, well I had no where else for my new fish so I put them in the tank and took me dead fish back to the pet store to get a refund and was told my ammonia levels were too high and when I asked how to fix this they told me my tank was cycling and that fish would die, well after an angel fish and another tiger barb died I decided to go to a different pet store and I bought a penguin bio wheel filter, a tester kit, and some rocks to get rid of the ammonia. Well my ammonia levels haven't gone down and I've just lost 2 green tiger barbs and my last tiger barb, leaving me with 1 angel fish who I have now moved to a make shift tank hoping that I won't kill him in there and now I have taken apart my tank and am looking to start over again because I want to put my angel fish back into the tank and also some black red tailed sharks I have in a smaller tank. So now time for my questions: how do I keep my ammonia levels down? should I just get rid of the undergravel filter? How often should water changes be done? how often should the gravel be cleaned? how do I clean the gravel? should I get rid of the gravel and go for larger marble type rocks? I think that is all the questions I have so I'm going to go continue to clean out my tank. Thanks a bunch