I'd cross off all the 18" width tanks. Too hard to get a good feeling of depth in the aquascape. I have a 24" high tank and it's a PIA to get to the bottom. Tanks more than 24" width will be hard to reach the back, especially the back corner with no side access.
You need to figure out some...
You might try feeding larger chunks of food instead of the chopped up FF.
When I had a LF, it liked shrimp. Regular old grocery store raw shrimp!
An eel I had loved whole Grunion. Silversides would be a good substitute.
IIRC, neither of these fish need to or should be fed every day.
Nice fish even if it does look like a black tang with a grafted yellow tail! :D
I'd do at least a couple of courses of Prazi and tank transfer to rule out Ich.
A roommate and I had two tanks in college. Plywood tank and cooled by an old Coke frig with a bucket of cold water and a hose coiled up in it. Local San Diego fish. Mostly just a BIG Moray eel.
The eel was caught by a fisherman and he didn't want it so we snagged it. Always keep a bucket in...
I'll add to the anecdotal reports. I had an acrylic tank with many small Chitons. After a few months the bottom half of the tank looked like frosted glass. Upon inspection with a 10x loupe the 'teeth' marks could be clearly seen.
IMO these are a pest. Beware!
Yes 700 - 800 is normal pressure. CO2 is actually a liquid in the tank and at room temp produces that much gas for 700psi.
If you want to stop gas flow completely shut off the valve on the cylinder. The big green knob is for adjusting outflow pressure.