Phosphate Reactors.

[i have crabs]

its the type of material in the reactor that has effects on corals. if its iron based and you dont rinse it well leathers and some lps i believe dont like it.
http://www.reefcast.com/downloads4.php
lisen to episode 1

as far as the reactors they work great i have 2 running of a mj1200 1 full of phosguard and one full of carbon and its made a noticable improvement since i started.