Please Help! Plec Dying??!!

[elf]

Quick background- 2.5 year old 55 gallon with Oscy & Plec each about 13"
50% water changes every week, steady temp & water forever...

Last night (3AM) Plec was sucking on the tank as usual but seemed to be sliding down. I looked minutes later to see him on his side.
This has been going on ever since! He will lay on his side or back, then swim around with some bubbles coming out, do spins sometimes like he's dizzy, bump into the top of the tank & "fall"/slide down -
But sometimes in normal position like nothing is wrong. He's breathing normal, looks normal, eyes normal-
WHAT IS WRONG???!!!

He has no symptoms! The ONLY thing is about a month ago the "skin" right underneath his sucker (what would be his "chest")-if he's sucked on the front & you're looking right at him-had a patch that looked scraped off. It even looked kind of bloody.
I watched & it healed back just fine! He never showed any symptoms of anything!

He still shows no symptoms, though that "patch of skin" has a whitish hue-as in you can see where it was, almost like a scar.

We thought he might be bloated, & the inside of his 'mouth" looks kind of red and we thoughiBut sometimes in normal position like nothing is wrong. He's breathing normal, looks normal, eyes normalBut sometimes in normal position like nothing is wrong. He's breathing normal, looks normal, eyes normal where his "bottom opening" did to, but this was in looking for something. It's not obvious stuff.

We put a bit of epsom salt (5 teaspoons) thinking/hoping it is constipation.

PLEASE HELP! WHAT IS GOING ON??!! He's been fine & ALL OF A SUDDEN...!!!!

THANK YOU!!!!!!!!!!!!!!!!

[Damon]

Tank specs please.