I have a pair of metal or steel shelving units and each shelf (there's 5 or 6 shelves for each shelving unit) is supposed to hold up to 350 lbs each. on one shelving unit there's 2 shelves that would be perfect for a tank. Even though its supposed to hold up to 350 pounds I'd only put up to a 10 or 20 on them (they're not big enough to have a larger tank on them anyway) and have a board underneath to even out the weight. Do you think that would be ok, since they were made to hold that much weight? The shelving unit is on wheels, does that make any difference?