A lot of reefers I know keep sumps between 50% to 100% of their tank volume, so a 20g would probably be good enough for your 39g tank. Of course, since sumps also add water volume to your total system, bigger is better. My friend has a 180g sump for his 165g display reef system.
If your sump is going into the stand under your display tank, then figure out what's the biggest size tank/sump you can fit under there & that will be your limiting factor.
Hope that helps,
Anthony
|