Okay, here it goes.
There was a time that i would have to clean out my goldfish tank everyweek. And one week after I took out the fish and cleaned the tank I realized that the only water I had was cold. ( I can't use my sink water b/c it has a high lead content) Well, I thought that it would be fine so i filled it up. I then realized that once I put the gold fish back that it was too cold. So I took the heater from my betta tank, rinsed it, and put it in the gold fish tank. After a while everything went back to normal and I never did that again.