Nitrates at 20 is okay for corals, they'll live through that, although as close to zero as possible is better. How much do you feed your fish? And what kind of food? Flake food and pellet food will lead to nitrate problems, if you're using those I'd switch to frozen, and feed them frozen only. Feeding once a day is fine, the fish will always act hungry. Other reasons for high nitrates is too many fish (and six in a 100 gallon tank is not too much), something died in your tank and is rotting (it could be a snail or something -- have you had any animals disappear?), overfeeding, and the water you're using. What kind of water do you use for top-off and water changes? Tap water can have nigh nitrates. You should use RO/DI water. Check your new water for nitrates before your next water change to make sure the new water you're putting in doesn't have nitrates. Also what kind of filtration system are you using? If you don't clean your filters enough, or if you're using a canister filter, you can have nitrate problems. And how much live rock do you have in your tank? 1 to 2 lbs per gallon is recommended, if you don't have enough live rock you can have nitrate problems too. Lots of possibilities, help me narrow it down.