He's a super long finned betta I can't think of the name for him but he was 15 dollars he's always been less active then most Bettas but recently all he does is lay down at the bottom of his tank his fins are frayed like fin rot. He's in a 10 gallon tank with 2 other Bettas which are all divided. The tank is heated and filtered and I do 25% water changes a week. Both of the other Bettas are acting normal. What could be wrong with my betta?