How big of a sump do I need?

[Gripenfelter]

For my 120 I have a 30 gallon sump and a 18 gallon refugium.

For my 300 gallon I was thinking a 70 gallon sump?

[parkinsn]

I would try to get the biggest sump you can fit under the tank while still having room to work. The more water volume you can add to the system the better.

[Lampshade]

+1 on biggest possible. Never heard people say their sump is too big. Lots wishing they went bigger, me being one of them.

[parkinsn]

Im a fan of long and shallow as of late. That is what I'm doing on my new tank. If your tank is say, 6'x2' try to get a sump thats 5'x18" or close to it. Hight dependant on your stand. Also try to leave yourself the ability to be able to slide the sump out if need be.

[Seamazter]

You might wanna consider an algae scrubber too, if your building your own sump.
Many have found great success with these, they are very easy to do, a simple Google search should provide you with great results.

[parkinsn]

Other things to keep in mind for sump's would be, ATO reservoir, refugium, doser reservoir's for your chems (if your using a doser or plan to), filter sock holders. The bigger/more custom you go the easier it is to have these things built in.

I will give you a rough idea of my new sump, dim's are not final yet. Its going to be 32Lx24Wx15H Im going to have a 5" strip 15" high running the 32" span for ATO. Im going to have a 5" strip 15" high on the "front" on the sump chamberd in 3 for Alk, Ca and Mg. the remaining 27"Lx19"Wx15"H will chambered into return, skimmer, filter sock intake sections with baffles between.

[Gripenfelter]

Algae Scrubber? Hmm. Anyone running one? Still not quite sure I understand how exactly this works.

[The Codfather]

My sump for the 300 is going to be 66"x22" and 24" in height.