How much to feed??

[Dawn II]

I have had nitrates in the 15-20 range for some time now. I do water changes every 2 weeks, for a while there every week (25%). I have been told that perhaps I feed too much, I cut back, and fish look thin. So, for 160 gal, (3) tangs 4-5", skipper dragonet (2"), lawnmower blenny (3"), firefish, (2) chromis -2", shrimp goby, (3) shrimps, 10" clam, (2)sand sifter stars, 12 snails. How much to feed, and how often? Give it to me in terms of frozen cube size, even though I feed other things, it will give me an idea. Also, how much Seaweed Selects should I hang daily- I/4 sheet, 1/2 ???