Calcium and dKh dropping rapidly!

[mrhasan]

So I always do a 5 gallon WC every saturday and don't dose anything. But recently, my corals have kicked off rapid growth (sps; for some reason, my LPS, other than acans, don't grow!) and birdnests are growing like weed.

So on Saturday, I start with SG 1.026, Ca 410-420ppm, Alk 9dkH (don't measure Mg). But the dKh falls very rapidly (today, I read something between 5-6 with API) and the Ca is resting around 380. I have not seen any issue with any of the corals (other than two zoas: one closed up and one opens slightly; I never have any luck keeping zoa - don't know why!). Alongside, I don't get growth in my torch (been one headed for like 5+ months). Funny thing! My SPS are growing crazy (birdnests; montis; red planet) while LPS are like...naaaah I wont grow!

Advice please Should I start dosing? Or should I just keep up with the 5 gallon change since there is no ill affect on the corals?