Hi, My platy has a small red bump/spot on his left fin. I think I see a bit white around it, but it's red in the center. He eats normally, there are no other fishes in my 5 gal tank. The water is OK - no ammonia or nitrites. The pH was low today, but I changed about 30% of the water right away. It doesn't look like fluffy patches. What else it could be? Thank you so much for your help in advance!