7.2 ph is way too high for coco, aim for between 5.2 and 6.2... around 5.8-5.9 is best. Get some ph down from the hydro store, or you can use lemon juice as a quick fix... but definitely get the ph down and some ph up also, meant for plants (not for aquariums!).

From now on, always ph your water before feeding. Surprised your other ones aren't having problems yet, but I'm sure they will soon also.

Forget the CalMag part since your using tap... Another good measure is to let your tap water sit out for 24 hours to let the chlorine evaporate from it. Might not be the problem, but it won't hurt to do it either.