I did that once with my old 110G FO tank. I wanted to get into reef tanks, but I coudn't put corals and inverts in ther because of my eel and triggerl. So I drilled the side of a 20G tank from Petco and made a stand for it next to my tank. I hooked it up to the same sump and It was doing fine untill I sold the tank. Although the water condition wes more stable that way, the problem for me was that my FO tank did not need all the dosing a reef tank needs, so basicalliy I was dosing for 130G when only 20G really needed it. Also the large fish I had produced lots of waste, too much for a reef tank.