Your suspicion is correct: they do let nitrate build up, and most corals really detest nitrate and just won't thrive. SOfties are more ok with it than stony, and hammer (a stony) tolerates it better than candycane (another stony) but ALL corals are happier without a nitrate overload. GO on rock/sand alone, with a moderately good skimmer. That's enough. You don't even need a really potent skimmer unless you're trying sps.