heating queries

How exactly do you do large water changes in a heated tank? I’m doing 75% water changes in my bettas tank right now as he has fin rot. I let treated tap water sit out for a few hours to get to room temperature (about 70 degrees F), but his heater is set at 78. He is sitting in a cup right now...

I have just taken over my dad's tropical fish tank and I am having so much trouble with the heater. I plugged it in and set it to the temperature he had it on which I think was 26? It was hard to see. Worked fine, water was lovely and warm. I put my hand in the next morning and it was boiling...

I currently have a 100L tank with 4 x tiger barbs, 4 x platy, 5 x bosemani  rainbow, 8 x neon tetra and 1 x bolivian ram. Had it for a year and quite happy but want to start keeping slightly bigger fish.  I’m currently thinking of upgrading to a 250L tank from All Pond Solutions.  ...