Cloudy Water Ideas

[ruslicus]

So over the week I had:
PH range 7.8-8,2
Alk 8.7 dropped to 8.3
Ca 450 dropped to 370
PO3 0.08-0.00 and back to 0.06 - I think my feeding affects PO3.
NO3 is 0.00 all week.

It is time to do waterchange. Should I go ahead and do WC, how much? should I try to dose and keep Alk and Ca consistent? How I can drop Alk to 8.0? I am using H2O salt at 1.025 salinity.
My pocci finaly is puffy, but I noticed my Powder Blue Tang has itch I think. I gave her garlic veggie

Thank you