a 20 would overflow with a power outage on a 150 gal i would invest in a larger rubbermaid plastic box there cheep and you can drill them easly if needed
im using a 18gal bin as my sump. works fine. just measure and test a few times where you need to keep it filled to leave enough room for poweroutages so it doesn't overflow.
been using it for the past 2 years with no problems at all. survived 2 power outages so far no spills
I was thinking about going with an overflow, because the tank is actually an indoor pond. I could build a waterfall or something with concealed bio material