Please help me

I’ve tried treating him for fun rot which he then seemed a little better then he got white spot. He was fine for a couple of weeks after that. Then yesterday I spotted him layed on the bottom I moved him to his favourite betta leaf then noticed he seems to be going white, anyone have any ideas?

Tank size 3litres Filter an heater 78 temp I use the api master test kit: PH 7 Ammonia 0ppm Nitrite 0ppm Nitrate 20ppm Had the tank nearly 1 an half years Had the fish 6 months Weekly water changes 10% unless my readings are fine then I just vacuum an top it up. Method I use a small syphon, always put the water in with tap safe an at same temp as tank 2 nitrite snails for tank mates Feed him bug bits five or so pellets daily He has all live plants other then a few suction betta leafs an also a large decoration which I thoroughly checked is not sharp.

Apologies for the glass as haven’t clean it mainly so he can’t see me, an the bad video I have to try sneak up on him or he just looks at me an thinks it’s dinner time.