Divide the tank into at least 2; maybe three. Cut plexi glass to size and slide it into place as a divider. Then use clear silicone to seal it up. You will have 2 or 3 tanks made out of 1. When ever you want to make it a 55g again; the stuff will scrape off with a razor and wipe clean with denatured alcohol.

Here's one I made into 2 tanks


Good luck!