Sick Gar - Need Advice

I have a Florida Gar that appears to be ill. The gar is 4yrs old, 20 inches long and is very thick (so it's not lacking reserves). About 4 days ago I noticed it laying on the bottom of the tank all the way up against the side. When I walked up the gar didn't move so I put my hand up against the glass to see if he would swim away. Nothing. It seems like he went blind. He has no reaction to a flashlight shining on him or any eye movement. When he swims it seems like he is a bit wobbly. He has been like this for 4 days now. His tankmates are 2 other gars and 2 smaller motoro rays. All other tankmates are perfectly fine. Can a gar have a stroke...does this make sense? Any ideas on what to do?

All water parameters are fine - 0 ammonia, 0 nitrites, 40 nitrate, 7.5 PH

He's gone. I went in the room and noticed him laying on his side. The white part under his belly was turning pink, his mouth was partially bleeding and he was barely breathing. I had to euthenize him as he was suffering. It's a sad day....