I use a pub cylinder I think it 13kg or more and it cost me £18 to swap for a full one ..I leave it on 24/7 now so I don't get ph changes so not sure how long it will last
I put washing up liquid round it and all other connections as there didn't seem to be any micro bubbles in the tank .. The lid is a one you turn just a quarter to lock it, the fluval looks like a screw thread lid which I'm sure would be a better seal
Found the instructions online but very vague ...it says it is adjustable but no further instructions on turning it up,
Don't want to turn that nut now as you said it could possibley be for spare dial......