Hello Ace, and welcome to the forum.
I'll start with a couple of questions--
Where are you located?
Are you growing cacti indoors or out?
Can you post a picture or two?
I've used the MG cactus/succulent mix, but just for succulents, and mixed with up to 50% additional grit. For grit, I've used well rinsed pumice, poultry grit (chipped granite), or pea gravel with good success. I don't care for perlite.
For potted cactus, I've switched to Bonsai Jack's Gritty Mix. It does not retain water, and I don't think it would support mold growth of any sort.
Fungus is everywhere. Soil fungus is not necessarily a plant pathogen. Treating with a fungicide will not, in itself, keep a cactus healthy. Your growing conditions are favoring mold growth, not plant health.
In my experience, desert cacti are difficult to keep as houseplants. They like long bright days, and hot (80F.+) dry weather. Even with heat mats, grow lights, proper soil and watering it's hard to keep them happy indoors.
Some do best by letting them go dormant in Winter, keep them dry and cool through the short dark cold days.
IMO I would not use peroxide on plants. It kills cells by oxidizing them. It doesn't know the difference between a fungus cell and a cactus root. I think it does a lot of damage. I recognize that others disagree. (Peroxide is no longer used for wound care, it prolongs healing.)
I hope you will hear more from the cactus experts here!
