Oh and as an aside, I would NEVER put one filter on a tank.
This is especially true of the mag250s because they seem to clog/lose their flow rate pretty easily. Because the micron filter traps so much the flow rate can slow within a very short amount of time, so I wouldn't trust one mag250 on any tank, even a smaller one.
Mag250s are primarily useful at water polishing IMO. We have a couple with the biowheels on our tanks but they're both combined with other power filters and/or canister filters.
We did have a mag250 on a 29g for the primary source of filtration at one time, but it's now combined with a penguin because of the flowrate/clogging factor. This is also a very lightly stocked tank.
The biowheels seem to stop very easily on these filters if you don't keep them clean enough and or the catridges aren't cleaned/changed on a regular basis.
Ok, I guess in short, NO , I would not put one mag250 on a 55g.