Betta Fish: Disease or colour change?

Hi, i just noticed these small black spots on my bettas top fin. he's otherwise healthy and active. he's in an 8 gallon, with all live plants other than betta hammocks. Because its so heavily planted i only do a water change once a month. I feed him twice a day, mainly fluval bug bites for bettas, but the occasional brine shrimp as a treat, i also do the occasional fasting day, though there are daphnia and baby snails he'll still snack on. water is at 26 degrees celsius, and I never have any detectable ammonia, nitrites, or nitrates due to all the plants. I keep catappa leaves in the tank for the tannins. he has a small hob filter that's turned down low, and buffered even more with filter sponge at the outlet as he HATES flow. Otherwise he shares the tank with 1 otocinclus (she bullied other ottos and loves being alone. I know you're normally meant to have more), and mini ramshorn snails. any thoughts on whether the spots are his colouration, or an infection?