I have a 29G with sand, live rock, koralia 2, Aquaclear 30 HOB filter with sponge, and a Current PC fixture with 2 dual 65W PCs; and I keep a ball of chaeto and some red macro algae in it.
I have it attached to my 120G system all the time and when I need to QT, I shut off the return to it and lower the water an inch or so below the overflow. Depending on what I'm QTing I do waterchanges every few days using water from the 120 system and once I'm ready to move it to the 120G display I open the return just a bit and let the system water drip back into it.