What in the world happened to my fish and aquarium ?

the fish and aquarium and fish about 3mos old. Three panda catfish, one Moore, one koe and 2 small goldfish, 2 other small type goldfish all seemly happy together. I saw two of the pandas floating on the bottom. Got them out and found the other one. Cleaned my tank from top to bottom, put it back together, added the water and water conditioner a few minutes with some aquamarine sold. Added the fish and within minutes two more went. Everything the ssame as beforethe fish died, I feed them small amounts? twice a day. What could wt

Don't use chlorinated water straight from the tap. Let it sit for a few hours and off gas the chlorine.

you should also use some of the old water too with the new and let it set out for 24 hours before you put it in your tank. Next time one dies-look at the tail. I had some beautiful goldfish and I picked up some new ones and put them in my tank and a few days later they started dying off one by one and I found out that the new ones I put in there had tailrot. Little white specks on the tail.

Fish can only survive if there is bacteria in the water. The bacteria can only survive if their are fish in the water. The two depend on each other for survival. If you start up a new aquarium, and introduce fish into that aquarium, a couple of them will die until enough bacteria exists to sustain other fish. This is why when you start up a new aquarium, you buy two or three cheap goldfish. Then after you have one or two survive for a couple of days, you can then introduce more expensive fish. When you clean your aquarium or replace the water, you loose bacteria. This is why they recommend that you only change about one third of the water during a water change. If you replace more than half of the water, you will deplete the bacteria, which may result in some more fish dying.

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