Beta fish fungus?

Fungus is actually quite rare in fish more likely it's columnaris. Betta seem to get this when stressed. I'd be sure condition are okay for a betta.

-Temp 75-85

-Nitrates under 30 ppm (You should really change 20-40% of his water every 2-3 weeks)

Also I'd consider a course of antibiotics. Ideally a broad spectrum, or at least one treating gram negative bacteria. I prefer trisulfa.

PS- The betta talk link talks fungus, but proscribes a tearment that works for both fungus and columnaris.

It depends on what you mean by sore looking as to what the problem may be at this stage. If you mean it looks a little red and angry but nothing more then odds are good it was a simple fungus that the fish has fought off. If you mean it looks like an open wound then odds are good it's columnaris and not a fungus at all. Columnaris is a bacterial infection that looks very much like fungus but of course being a bacteria doesn't respond to fungus treatments. It often starts on the head or around the mouth of the fish, hence the common name of mouth fungus. If it reappears in the next few days start a fungus treatment asap. Personally, I prefer Maroxy for a good quality fungus control. If that does get rid of the cottony looking material within 2 days I would stop using the fungus treatment and start an antibiotic. The best antibiotic for columnaris is Furanace. If that's not available use tetracycline, such as Maracyn TC. Just treat as directed on the package.

MM

Is the water too cold, or does it change rapidly from night to daytime? He must have been stressed and developed some kind of fungus. I'd go buy some medicine and put him in another tank for awhile. Don't mess up your tank. It has developed all the good stuff and it takes awhile to get it back.

Can't you buy some magic solution at pet smart?

by the way, your mom's a beta fish fungus.