Another problem is most display tanks are glass; which is a bit more difficult to work with when adding baffles, etc to turn the glass display tank into a viable sump tank.

 

A 40g breeder tank is one of the better sizes for sump conversion; it's height is not too restrictive and if you use glass baffles it can work well.

That's the beauty of the modern propeller pumps pushing massive, healthy amounts of water within the aquarium. The built-in overflow leading to a reasonable size sump/Refugium typically cannot handle, more than 1000 gph. That sort of flow(1000 gph) easily turns over 5x the water of a 210 gal main display in just one short hour. Do the math. For a 210 gal aquarium with LR and LS, that's ~120x turnover per day. More than enough to handle the supplemental filtration activities of a Refugium, a Skimmer and bags of AA and AC placed near the return pump intake screen. 

Remember, more than 85% of filtration in a typical reef aquarium occurs in the LS, LR, LW and algae growing within the display. High water flow within the tank provides the conditions where this live biofiltration can thrive.

The development of the propeller pumps has helped in three important ways regarding the average hobbyist tank(<300 gals).

1) Healthy water flow, somewhat approaching that of the natural ocean can be attained;

2) The use of massive return pumps and associated overflows has been eliminated; and

3) The former "closed loop circulation" technique with it's pump/piping and numerous holes drilled into the glass has been eliminated.

Aloha,

Mark  

808-345-1049