Factors influencing Alk depletion rates?

[TimT]

Alkalinity depletion is a sign of growth so I think it safe to assume that the growth has stopped or been greatly reduced. Some thing has either disrupted it or a nutrient is so low that it is limiting it.

I have found that temperature fluctuations(~3+°f) will cause a disruption.

pH out of range of 7.8-8.4 can have an effect.

Have you tested your water for copper? As RO membranes age they begin to pass more copper. Some houses still have copper pipes.

What is your salinity and has it changed recently?

Your Ca seems a bit low. I would recommend at least 400ppm.

When was the last water change? Was the salt mixed to redistribute the chemicals before being added to the water? Have you switched salt brands recently?

SPS do rely on zoozanthellae for energy and the zoozanthellae rely on nitrate for food. Having 0 nitrate could be a limiting factor.

Just some ideas.

Cheers,
Tim