I'm sorry if I'm posting in the wrong place

We have a sick plec and I don't even know what kind of plec he is. You'll need to bear with me, per my username, I'm a bit out of my depth here as the plec actually belongs to my other half and I'm clueless!!
He's had this plec for at least 6 or 7 years (that gives us an idea of his age lol) and he's been acting strangely the last week or so. He's lying on the bottom of the tank and I haven't seen him swimming or anything for a while. He's usually such a busy guy!! He's twitching/jumping frequently and hasn't moved positions for a few days now. The water has been tested and is showing nothing unusual. I'm sorry, I can't be more specific than that

He was on his side at one point and seemed like he couldn't turn himself over so swim bladder was suspected. Two lots of the treatment has been put in the tank but he doesn't seem to be getting any better.
I can't even be sure he's eating and I can't see any evidence of poo! (Sorry, tmi!!)
Does anyone have any idea what could be wrong with him and/or how we should move forward with him? I really don't want to leave him like this if he's likely to be in pain.
Here's a pic of him, in case it's of any use. Sorry, half of the pic has been cut off!

Thank you for any advice

And apologies for any rule breaks I may have committed
