What would be some tell tale signs of low Magnesium? Besides my 90 gallon I have a 60 cube. Past couple of weeks I have been having bubbles on my rocks, bubbles come up from the sand. And thick rust colored brown stuff on the sand. That stays clumped when you siphon it out.
the brown bubbles are dinoflagilates. I believe (and someone can correct if I'm wrong) this is caused by high silicates in the water. Are you using Ro/Di???
One sign of low Mg levels is that you will have a hard time growing Chaeto, mangroves, micro and macro algaes... as they all uptake MG for their bio process.
if after confirming tests show low MG. you could just do several water changes as IO salt seems to be high in MG. You could also go to your local CVS and pick up pure epsum slat which is a pure Mg supplement. careful not to overdose here. Use randy's calculator to raise to desired levels.
When my mag got low my alkalinity was all over the place. I was trying to dial in my dosing again. My alk would go up by 5 ppm one day, then I would drop the dose by the appropriate amount to keep it steady. Then test the next day and it would drop 10 ppm.
Checked my mag with a new test kit and it was at 1200. Raised to 1380 over 5 days and now my alk is rock steady again.