problems with millaporas

[Amadod2]

Tank size 75g , 3 powerheads jebao wp40, tunze 6065 and a korelia1150, the other corals are doing well i will post some pics. im not using carbon, only rowaphose, and the corals are located mid tank, i started my tank in august and my first water change was done in december, but the millaporas were ok, i also was using tap water for top offs, now im doing water changes every month 15% and using RODI water for everything