I had the recommended amount for the tank in! Just freshwater salt, I guess.....
Okay, so now I'm confused. I had those mollies die, and then I bought two female bettas, and a catfish to go in there. A little Cory catfish. The Cory died after about a week. It had this weird red blotch on it...!?!?! And now one of the female bettas turned up dead. And a Danio that is 2 months old has died. !?!?!?! What am I doing wrong?! Should I take the water in to be tested?! Also, these fish never showed any signs of being sick before upping and dying. :/
On a brighter note, my fantail goldfish are doing AWESOME!!! So happy and healthy!
You could be having one of the bettas murder the other fish. The females can be just as aggressive as the males. Actually, it is NOT recommended to try multiple females together unless you're a very experienced fishkeeper.
Cory catfish should be in groups of 5 as well. Less than that and they get easily stressed and can die. How big is your tank? Are the fantails in this same tank as the other fish are dying in? Because bettas are tropical fish and need much warmer water than goldfish in order to thrive. Goldfish are also huge ammonia producers and need very large tanks and loads of filtration.