I am in a rush so I'll try to do this as fast as possible while still being thorough.
I have two bettas, both in side-by-side heated/filtered/planted/cycled 5 gallon tanks. Walter is a red/blue veiltail and Oscar is a blue doubletail. Ph in both tanks 7.5 and temperature 78 to 80 degrees Fahrenheit. Both live with a zebra nerite snail.
All was good until I went away for a few days and came home to find Oscar laying on the bottom, his fins flared out and breathing rapidly. He has just sat there for 4 hours and when I poked him to make him move, he got up but just kept shaking his body spasticly and he flashed against all the plants again and again repeatedly for about 5 minutes like he was very itchy. He isn't eating either.
No ammonia, no nitrite, nitrates 10.
No new fish or plants to the tank since 2 months ago when I got the snail.
I am sure it is some sort of parasite, but it isn't ich or velvet. Long ago I had a betta behave kind of like this and it ended up having anchorworms which killed it, but so far Oscar has no wounds nor worms visible and other than his behavior appears healthy, with good color and spread out, undamaged fins. The snail is behaving normally. What treatment would anyone recommend I get based on his symptoms?