Personally, I think you've got no chance of getting those screws out, even if you move the tank a bit, as they are Phillips heads, and there still won't be enough room to get to them. Whoever came up with that method of fixing them, needs shooting! It'll certainly be a right pig, and could take you hours! If it were me, I'd carefully break the existing lights off, just hacksaw them up a bit, so you're left with the stud and hex nut poking out, and then use a cutting disc on an angle grinder and slice them off.
Oz,