If you're serious about this, here are a few things to consider. If you're looking to vertically lift a 48"x 24" or larger cover, and you don't want 'ugly' assemblies at the two front corners, you're talking about a mechanism which attaches at the two rear corners only. This mechanism is not only going to have to handle the vertical lift, but it will also have to handle a fair amount of 'torque' because the tank cover appears as an unbalanced overhung load to the mechanism.
To support the cover weight and torque without bending problems, you're probably talking about something like two 1"x 3" aluminum square tubes, running from floor to ceiling behind each corner. You'd then need a cover attachment assembly with two pairs of 1"+ wide flat bottom rollers spaced about 24" apart to 'ride' on each square tube. You could put say a 4" diameter U groove pulley near the top of each square tube, with cables (i.e. small plastic coated steel clothesline cable) starting at each cover attachment assembly, then going up over the top pulleys, then down the back side of each square tube, and both cables finally attaching to a common winding pipe (maybe 2" diameter) set on two bearings (one bearing mounted near the bottom of each square tube) and driven by your gearmotor via a timing belt pulley at the center or either far end of the winding pipe. You need to use a common winding pipe or some other single mechanism to make sure that both corners always move together in unison, or the mechanism will jam up. Also, be sure to get a worm gearbox so gravity doesn't take over when you shut the motor off ! Fairly cheap and not too ugly, but the cover attachment assemblies would take a bit of fabrication. Many parts could be made of PVC or acrylic though, which would allow for easy 'machining' and glueing which might come in particularly handy for the 8 flat bottom rollers with extended flanges needed to 'ride' on the square tubing.
If you don't like the idea of cables and pulleys, the same basic idea would work with a gear rack and pinion set halfway between the two square tubes, but then the gearmotor would have to move up and down along with the cover. You'd also probably want to think in terms of a brake-motor because you don't want the whole thing coming down on you when you're bent over the side of the open tank! You could also use a pneumatic cylinder halfway between the two square tubes, but it's difficult to get a smooth motion and you need a compressed air supply. You could also use a hydraulic cylinder halfway between the two square tubes, but then you need an oil pump and tank and valve plus having to deal with the mess when oil leaks develop (and they always do).
Just some ideas
~
