Before you go through all that work why not one of the many excellent anti-fungal products on the market?
My black moore was sitting on the bottom of his tank with white over his eye and everything; at death's door. I put in some Jungle Labs Fungus Eliminator. That was four days ago and my guys are both swimming at the front of their tank, wagging their tails, going 'feed me, feed me'. I don't see any white left.
It will be much less stressful to him than a complete tank changover. Good luck with your beta.

The best and easiest way I know to clean any tank or equipment is with bleach in fairly warm (about 110 deg.) water and if there are any lingering bits of organic grime, adding hydogen peroxide to the mix will "boil" it out without harming the material and mostly eliminating the need to dig into tiny slots that mfg. love to add to their filter designs.

However, fungus can't live on a clean, dry surface and as sunlight alone is a very effective anti-fungal treatment, and you have already done all that work, just rinse it very well and set it out in the sun. By the time it's dry, it will be as sterilized as you can get it short of melting it in an autoclave.