no need to get a power filter. It's a plan to get detritus trapped in it. The skimmer + live rock + powerheads will do all the job. BTW don't buy a seaclone protein skimmer even if the price looks right and the LFS smiles saying it's great.
If he does, go to another LFS
ro/di = reverse osmosis / de-ionised water. Well it really means super filtered watered with something like this :
Like...99% of the crap in the water is removed. Tap water from the faucet may contain silicates, heavy metals, nitrates, phosphates...wich aren't welcome in a SW aquarium. Nitrates, phosphates and silicates are great to get unwanted algaes. Tested. Ok now let's say you have the unit, you have your filtered water. You can't put it in your tank right away because usually the water is more acid than the 8.2-8.4 you are supposed to have in your tank. The buffer increases alkalinity and ph. BRB i'm busted at work !