Colt corals

[Maudsley]

I have 3 colt corals....water parameters are Alk, 9.6DKH, Ca 500ppm, MG 1350, ph 8.3, temp 78 salinity 1.025,phosphates undetectable, nitrites 0, nitrates 5. All other fish and corals are doing good but the colts seem to be laying over. They were huge and now seem to be shrunk a bit. Any help would be appreciated. Last week did a 40 gal water change using IO and RO/DI water, I'm assuming my iodine is fine since the water change was just done.