4 of my 5 fishes have suddenly died this morning almost at the same time. I took some water to petsmart and all the water levels were out of range, but it's probably been like this for a while, I doubt this would be the cause for the sudden death of the fishes. I had one otocinclus catfish, one red wag platy and 3 zebra danio. Only the red wag is alive. Last Wednesday someone gave me a used but still working heater, and the water has been at about 79 F since. He also gave me some decorations that I rinsed and put in the aquarium. I just changed about 75% of the water, added some ph increaser, ammonia remover and water conditioner, and some salt. What should I do? Thanks!