I have a 50 gallon breeder tank as my sump/refugium and it fits nicely in the stand I built for my 120 gallon tank (4X2X2). It leaves a little room leftover for the light, fan, height of the skimmer, etc. What are the dimensions of the 55 gallon tank in your garage? That would probably work for you instead of having to buy a used 50 gallon tank.