I think I figured out why my fish have been dying off. The well water at our house has softened water. Years ago when I used to keep bettas I had no problem with using the house tap water (always with water conditioner of course). But in the last past three years our house was set up with a water softener cause the water was coming out brown. Now when I try to keep fish they die in a few weeks of having them. I figured that was the problem cause when I kept the first fish, he was fine with the bottled water I was using. Then, when I switched to tap (with water conditioner and gradual switch) he died about three weeks later. And it happened to another fish only faster cause I was treating him for fin problems with aquarium salt (salt from the water softener + aquarium salt for fin rot = bad results).
So I just wanted to make this post for people that may have had similar problems. Our house water is softened using sodium chloride so I don't know if the same is true for potassium chloride. The aquatic plants were doing terribly in the water too.
Now I use bottled spring water.