small fin problem

[bettablue]

for the past three days I've noticed a really tiny spot on my bettas fin... It hasn't got any bigger but now he seems to not use it once and a while and "rests" it while he swims. I have a 5 gal, filtered and change half a gallon every week. last week I changed a gallon because the snail I put in there kept pooping so I took him out and then the next day the filter was being noisy so I added another half gallon.... I have added a teaspoon of aquarium salt... what else should I do? I would post a pic but it so small you wouldn't be able to see it...