Betta losing fin length?

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I have a male betta. he is alone in his own little 1 gallon tank. there is only an underwater gravel filter and his airstone. i change about 1/2 of the water about once every 2 weeks. and a full tank cleaning about 1 a month. he eats well, and doesnt act like he isnt feeling well. but the other day i noticed that his dorsal fin looks shorter. and his tail fin is kind of squared off. im pretty sure his dorsal fin was long enough to drape over his body and im positive the tail fin was not squared like that. my question is...do betta's regularly "shed" (only term i can think of) parts of their fins like that? or is this something else??