I agree, 20% should be fine as long as you have a skimmer and don't overfeed or overstock.
I also have a 6 foot tank and I use this ro
75 GPD RO/DI 5 Stage *PLUS* System - English
I was using an ebay unit but it was smaller and portable, I recently went with the larger and more industry standard that i linked.
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.