i have a 15 gallon tank with what used to be 10 fish in it. 8 of them were tropical and two are albino catfish. four fish have died(three of the same type). the reason the first fish dies was a slight ammonia increase (0.25 ppm). i changed the water and the ammonia disappeared. i don't know why 3 others died. in the course of two weeks. currently ammonia is 0, nitrite/rate= 0, ph=7.0, chlorine=0. any ideas as to the problem?