Dorsal fin dissappear

Hi, my betta's dorsal fin is disappearing he is in a 5 gallon tank, I clean it monthly, the water parameters are fine, he makes bubble nests, and his only tank mate is a nerite snail. He flares at the snail a lot, but never attacks it. His tail looks short, but not frayed, and his bottom fin is a little frayed. He's still active and seems like he's swimming fine.

Can you post the actual numbers from all your tests, please?

You should really be doing weekly partial water changes, of between 25 and 50%.

When you clean the tank, how do you clean the filter, exactly? How long has the tank been set up? Can you see any redness, or any white or 'fluffy' looking patches or edges to the fin?

Sorry for all the questions, bu the more information we have, the better we can help your fish

Nitrate between 0 and 20
Nitrite 0
Hardness 150
Chlorine 0
Alkalinity between 0 and 40
pH 7
Ammonia
I didn't know I was supposed to clean the filter, I change our the carbon pads when I clean the tank. I've had the tank for about five months. There's a little bit of white stuff on his head.

It's actually good that you don't clean out the filter; that's where the good bacteria that you need to eat the fish's wastes live. You can gently rinse it out, with some old tank water, if you think it might be getting clogged up.

It would be helpful to get an ammonia test, just to rule that out; everything else looks fine.

What sort of decorations do you have in the tank? Betta fins are very delicate and can get snagged on anything that's rough. I recently had to take all the wood out of my own betta's tank as he was managing to catch his fins on it; I've just got smooth stones in there now. Fake plants can be very rough too.

5 gallon tank, I clean it monthly,

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You need 50% weekly water changes.

What chemicals do you use to treat the water.
Has your tank got a heater?
What is the water temp right now?
What do you feed him?
How strong is the current in your tank?
Can you post a pic of the fish?
Can you post a photo of the tank?
What treatments have you tried? Give as much detail as you can.

I have a filter that is set so it isn't too rough, and a heater that keeps it at 78. I have a plastic cup that I put my snail in when I cleaned the tank last. I left it in a tank for a hiding place, but I'm trying to find something safer. The ammonia is at .25.