With 1" overflow at both ends and strainer you can have 1900 gph
return pump. You can always reduce the rate of flow by installing a ball valve on the return pipe. A 20 gal
sump may be a little bit small but with the ball valve you can upgrade any time. I have a 10 gal
sump on my 60 gal
DT and so far so good. If you can elevate the other 20 gal
for refugium higher than the sump, use the same set up as DT for overflow and install a Y on one overflow from DT as a splitter then provide a 2nd ball valve to provide a low flow for refugium.