Betta Fish: betta losing fin?

I’ve had my betta for nearly four months now but recently he’s started losing his fins and I don’t know why, he’s in a 40 litre tank with lots of live plants. he’s in there with a couple of corydoras and two glow light tetras, which have been there with him the whole time so i know it’s not them nipping his fins. any advice on what to do would be very appreciated, this is my first betta. i have tested my water regularly and all the levels are perfectly fine