Sinking fish - swim bladder issue? Need help please

Hi guys,

I'm having a problem I've never encountered before. My young Firemouth is sinking and sitting on top of the sand in my tank. Just sinks and sits upright on his belly. He is still otherwise acting normal. Eating, swims from his spot but its now a darting motion instead of 'swimming' per se. He can still get up into the water column but I can tell this takes effort as he's flailing his pectoral fins rapidly.

Water parameters are fine, no ammonia, nitrite, very low nitrate if any.

All other fish are doing well. There are no other Firemouths in the tank so I've ruled out that its some sort of nesting behavior.

I did notice a small white fungus looking splotch at the back of his dorsal fin where the fin meets his body. Its very small though. Considering spot treating it with peroxide if it doesn't clear up on its own.

I'm at a loss. I'm assuming its an internal infection or swim bladder issue? Also, he is not bloated, no dropsy, no popeye, no swollen abdomen or anything. Otherwise looks completely normal and like I said is still eating aggressively.

I've begun feeding Angels Plus medicated flake II. Not sure what course of action to take.

Can someone help me please?

I also took a video and will try to post it here if I can.

Sorry - have been unable to get back on line. I've fasted the fish for 3/4 days now. Still no bloating, still alive and otherwise well. I plan on blanching some peas tonight and seeing how that goes. All other fish are completely fine so I'm hoping its not a parasite. No new fish have been added and these guys have been together for several months. This Firemouth is a tough SOB too and has survived some crazy accidents so I'm not sure what's wrong... he's also just starting to put on some size and beautiful color. I'm hoping the fast and peas relieve him if its pressure on the swim bladder but I really am at a loss