Betta Fish: My betta is sinking

Saturday morning I went to feed my betta mochi and he was at the bottom of his tank on his side. I noticed too late and had already dropped a pellet in the tank and he swam up and ate it but collapsed right back down. At first, I thought swim bladder and moved him to a shallow tank with epsom salt and stopped feeding. I’ve been monitoring him closely and at times he’s motionless, only moving when I move and accidentally startle him. He’s either breathing rapidly or not at all for almost a minute. randomly he’ll swim around slightly then collapse, or tread the bottom on his side. I checked the water levels and everything was normal, heater functioning properly. It’s now day 3. The first photos are from a week ago, vs today. He’s actually a little more upright than he has been, mostly been completely on his side. (Recent picture not best quality.)