More fungus problems

[Auratus]

I do water changes every 3 days on my bettas. Last week I took apart my 3 gallon hex and washed down the tank walls, rinsed the gravel, etc. I saved about 1/3 of the tank water and added it back into the tank. 2/3 of new fresh water. This was 3 days ago and since then my betta that was in it has come down with a huge case of some kind of fungus.

I have this problem with a LOT of my bettas. I think almost every one has had fungus and/or fin rot at one point. So, my question is could there be something in the water that causes this? They only get it once I do a large water change. Then everyone suggests to do water changes every day which I think only adds to the problem. I don't have a test kit at the moment but I'll be getting one on Saturday. This fish is pretty old, but I don't think that's what is causing the fungus. What could it be...

 

[Nisha]

It might be just the stress the fish go through adapting to the new water and it's temperature.
It might be better to do more partial water changes rather than a huge change. That's what's worked for me so far.

Hope this helps.