Why are my aquarium fish dying one after the other?

The temperature could be an issue. Do all of the tanks have heaters? If not, room temperature water is usually not warm enough for tropical fish. Cichlids tend to need water between 76 and 82 degrees, depending on the variety. I usually keep my Convict cichlids at around 76-78. Other freshwater tropical fish tend to range between 72 and 78, with 74 being about good for most of them. Mollies tend to like their water slightly warmer, and they also require a small amount of aquarium salt. Since your problem is in multiple separate tanks, it seems unlikely that disease is the culprit. Water quality seems like a much more likely problem. You should have your tap water tested to see if it contains any chemicals that are harmful for fish. You should also be using a water conditioner every time you add new tap water to your tanks, since almost all tap water contains chlorine and trace amounts of heavy metals. Starting with temperature regulation and water quality control is probably your best bet. If you still continue to have problems, then you might want to look for other issues.

Well there are endless possibilities, I will list some of the most common that people usually overlook: *You could over feed your fish... (Fish don't need to eat as much as we do. I feed mine once a day with very little food, they've been living like this for years) *Make sure they have enough oxygen.. (Fish also need oxygen to survive, less of this will be fatal for the fish...Non-stop powerful water filtration or air bubbles unless its feeding time) *Clean everything before putting into the aquarium.. and make sure what ever ornament you have in the aquarium does not poison the fish like metal that can rust! Go plastic, ceramic, etc.! *If you are having fresh water aquarium, too much chlorine in the water can be harmful to the fish (most states have lots of chlorine in their tap water). Go to Walmart and buy some chlorine neutralizer! Good luck.... "Fish don't just die, just killed"

The way I see it, there are 3 possiblities. One is that there is some sort of disease spreading through the fish. Another is that the water might contain toxins or other harmful substances. The last one is that there are harmful parasites or bacteria in the water or in the fish.