Yes you do need the overflow because in a SW
tank, the DOC
are attracted to air and gather at the top of the water which needs to be taken into the overflow for the skimmer to skim the water properly.
I just want to add that if you think SW
tanks are a piece of cake, well they're NOT. It's a lot more work and $$$ to have it than FW
. If you're not prepared to spend sweet dollars for the "fancy stuff", you might as well save yourself a lot of headaches and money. Now I am not saying it can't be done BUT you're gonna have to pay for a lot more than just "live rock and fish". BTW
, you don't need a filter for SW