I'd start by fixing the water situation.
You need tests for alkalinity, calcium and magnesium. I use Salifert because it gives numbers.
You need reef salt, if you've been using anything else: Instant Ocean is good moderate level salt.
You need supplements for alk, cal, magnesium. I use Kent.
If your water is deficient in magnesium, start with that. Dose only so much as the instructions tell you, wait 8 hours for it to dissolve and work through the system, then dose again. When your mg (mag) is up to 1300, stop dosing that and test your alkalinity. Do the same routine until your alk is 8.3. Never mind the ph.
Then do the same thing with calcium, until it reads 420. DO IT IN THAT ORDER.
If you have stony coral, you will need a calcium supplement, because it will eat up more calcium than your salt mix has to offer.
RUn these tests weekly. If that's not enough, run them more frequently.
If your corals are taking more than a teaspoon of calcium supplement a week, you need to put kalk into your topoff, and you can ask about that when you need to.
YOur skimmer is SUPPOSED to pull black stuff. That's good.
If you can see white skeleton on any coral, that's not good.
Test your nitrate. If your nitrate is higher than .2 ---do water changes until you've lowered it at least below 20 and ideally to the point where you have to do the sensitive version of the nitrate test to detect it.
Before adding the biopellet reactor, ---try the water changes. They'll work much, much, much faster.