Two bettas die in the same tank?

[futureteacher]

I had Draco for about 8 months. He was perfectly healthy all the time, then about a month ago, following a water change he got sick and died. I have two other bettas in the exact same size and style of tanks and had cleaned both of their tanks on the same day and they're both fine still. Anyways I bought a new betta after cleaning Draco's tank and put him in there with new leaves and a new cave. Anyways this new betta was also pretty healthy. He was strange and would bury himself beneath his rocks, but once again following a tank cleaning he got extreme fin rot. I tried to get him better using aquarium salt and melafix and he seemed better, but tonight he died as well. My other 2 bettas are doing perfectly fine so I'm kind of confused about why these two bettas died. It can't be the water since the other two are fine, but they both died after I cleaned the tank. I'm wondering if the tank may be carrying something and if so how could I fix it? I don't want to put other bettas into the tank for them to die as well and I'm really upset over these bettas deaths.

I'm not really a betta pro, but I've had one betta perfectly healthy for 2 years and my other has a couple of problems from before I got him and I've kept him happy and healthy as he can be for a year. They're both thriving and both of these guys were fine until I cleaned their tanks. I figured I should ask here since everyone probably knows a bit more about raising them than I do.

[Pataflafla]

Was there anything that you bought brand new with either of them?

I know I bought a brand new heater and it almost killed my favorite boy because it was leaching chemicals into the water.

Is there anything that's been the same in that tank for those two bettas?

[futureteacher]

Originally Posted by Pataflafla

Was there anything that you bought brand new with either of them?

I know I bought a brand new heater and it almost killed my favorite boy because it was leaching chemicals into the water.

Is there anything that's been the same in that tank for those two bettas?

Nothing other than a new cave and leaves for the new guy.

[futureteacher]

Oh I forgot to add, my roommate also bought a fish, not the same breed she got a king and Monet (the new guy) was a crowntail. We both bought them from our petco at the same time and her betta also died about a week ago as well. So we're also wondering if there may have been something wrong with that whole batch that petco had?