2 of my half moon bettas have some tough fin rot. the black one (cosmos) had it severely a while ago (https://www.fishlore.com/fishforum/showthread.php?t=199446) but I got him healed up and put him back into his 10g home with his buddy who was also showing signs of fin rot, (shredded tips, rips down his tail, and I could see the bone part in his tail in a few bits) I figured since the tank was cycled and clean that I could put them both in there and treat them both for fin rot but something happened and the tank is showing low ph among other issues. note I did a water change yesterday and put new charcoal in the filter. there isn't anything sharp in the tank and the filter hardly makes a ripple as its pointed at the divider and a plant. the tank is heavily planted and there's also an abundance of white algae looking stuff on the glass? any idea what this could be? the temp is 80 it's hard to get pictures of them cause of the glass and because they scoot around super fast neither of them show any signs of heavy distress, they eat fine, 2 pellets a day each, and are very active. I'm going to get aquarium salt today but any help would be greatly appreciated as I'm leaving on Sunday for 3 weeks and they will be in my parents care.