Water Pump?

Is there a reason you don't want an external pump?

Water through the refuge area should only be about 5x and I would control with either a ball or gate valve. In fact I am going to be replumbing mine so that all of my overflow goes directly into my skimmer section and that the refugium area will be fed with the left over water from my mag 9.5 that is flowing faster than my overflow. (my overflow is rated at 700 supposedly, I don't think I'm getting that, and my mag is rated at 850 for the height difference that I have). I might upgrade the return pump a little higher depending on what actual flow I achieve with my new plumb job.

So to answer your question I guess I need to ask one, what is your overflow rated at? Then you can really answer your own question based on that and how you intend to plumb the sump.

I had redone mine based on the way the previous owner had it plumbed but have come to find out after much reading and chatting that he had done what I feel was a poor implementation simply due to the fact that I have to have the ball valve on the return turned under half flow which is putting a lot of back pressure on the pump. The new way I intend on plumbing should completely allieviate all of that.

What do you plan on putting into the 55, I have Melev's sump f which is a 40 gallon and have more than enough room for a return, skimmer and refugium.

You need to run a external pump and draw water from both tanks(with a t)or have the 20g drilled and flow it into the 55g,but I would'nt suggest this method because you typicaly want slower flow through you refuge.I have a 150g with a 29g refuge and a 40g sump,but I use a external pump,inlet to a "t",one side to sump,the other side to refug.I have a 600gph overflow going to refug,and a 1200 gph overflow going to sump.It works very well