Betta Fish: i’m worried fishlix might have fin rot, i’m unsure how to use kanaplex.

i ordered it and it arrives today, i’ve had him for 4-5 months and recently his fins have looked a bit icky and earlier he looked VERY off and had a dark spot on his stomach, but after he moved it was gone so i think it was a shadow, he ended up being fine and it was gone seconds after he moved. how should i use it to treat his fin rot? i heard putting it in his food is very effective, i feed him frozen brine shrimp so how would i go about that? ive never had to deal with fin rot before, he’s the longest finned betta i’ve ever had actually so i probably should’ve already had it on deck anyways but i never rly thought about it. he has no discolouration on his fins, but a few spots do look pretty jagged and he has no decor that could cause that. it’s all live plants and he has one hide that’s artificial which has no sharp edges and a floating lotus pad i got him that he uses the flared edges for mini bubble nests. he is also on a diet, idk if u can tell in the picture but he’s very plumpy so ive been feeding him every other day instead of every day and made preproportioned mini cubes for him and my other fish, he gets one mini cube. (it is a diluted amount of brine shrimp in the cubes.)