My beautiful betta isn't so beautiful anymore!?

Bought a gorgeous turquoise delta tail betta from the store earlier in March. Did a water change on him (not his first one, have been using recommended tap water conditioners) and the next day his gorgeous fins were all tattered and brittle looking! This little guy used to puff up and show off every chance he got. Still eats, still is very attentive to his surroundings, but he looks rough. I have removed anything hard in the tank, so all he has is the soft fake plants. Has anyone else had a betta with similar problems? What can I do to help him heal up? Thanks for any info you can provide!