Why are my Betta’s fins curled and maybe spotted?

I bought a Betta for my daughter that has quickly become my 5th child. We bought him from Petsmart, as I knew little about where else to go – especially with Covid19. He is in a 3.5 gallon (13 liters) tank with a filter that was advertised specifically for Betta fish. I conditioned the water and added a heater.

When we brought him home, his fins were curled up though they seem to be releasing a little more now but not fully. He seems to dart around a bit and then hid behind his filter though he will come out once and a while and be playful. Its been 5 days and I’m worried he might have issues with his fins and if there is something more I should be doing. His fins have a few black spots on them. I will get a water test kit tomorrow to ensure we have the levels right. Just wondering what you guys think. Any suggestions?