I had a thirty gallon fish tank. I changed the PH depending on the fish because some of them prefer a higher ph and others like lower ph. I know that with goldfish, you have to watch out for to much ammonia. I've had fish on and off for years, without luck. I'm not sure about the filter, I had the guy at the store help me pick it out. As for the guppies, they did breed fast but she never had more than eight in the tank. As for the idea that some fish aren't hardier than others, that's simply not true. In a tank with baja sharks, angelfish, and sucker fish (pleco?), angelfish always die first, then baja sharks, and then the sucker fish. Some fish have a greater degree of tolerance while some fish are downright persnickety.
