I have a 40 g reef with a 20 sump/refugium. A skimmer has a pump that creates a foam of bubbles and "skims" organic matter from the water. Water changes achieve the same purpose and also restore calcium and other minerals at the same time. It just isn't efficient to do large weekly water changes on a large tank.
A skimmer also has an added benefit of helping aerate the water. But I think you'd be fine with weekly 20% water changes on a 20g as long as you aren't overstocked. Live rock and powerheads will handle your biological filtration and aeration needs. I started with dry rock.
If I had to do over I wouldn't add coral until 6-9 months after the tank stabilizes a bit. The hair algae and cyanobacteria outbreaks in new tanks can be brutal as you probably know and it is no fun dealing with especially with corals.
As others have said, you will want a rodi
unit, I have the inexpensive "rodi
buddy" sold on Amazon for under $100 and it gets me to 0 tds
. You may also want to consider an auto top off unit a stability is key for coral happiness
I have some hammers and candy cane / trumpets. Duncans are popular easier lps
as well. I just run a black box 165w led
like Brookster mentioned and it works okay. You can find them on Amazon for $80+.
My own personal reef struggle is that I have bubble algae that I just can't get rid of, which is an argument for good quarantine process