Our sump is only a 10g. We would have really liked a 20g but, could not fit it under the tank.
Even though a refugium would be better, you can grow your macro algae there, we top off using the sump and if needed add any supplements from the sump.
You can also use the sump to grow pods and if neccessary throw any undesirable creatures in there.
We also noticed that when we added the sump it cut way down on those micro bubbles.
If your sure that you will be going with FOWLR then lights are not as important but, good luck with that! Most people I know have ended up getting corals. Too hard to resist!!
