Why does my sand turn to rock?

[flyboysmb]

I am posting this in this forum because I am guessing it has something to do with chemistry, but I am not sure. I have a 30 gal show tank with ~15 more gal in the sump. It has been up for 10+ years, only 3 fish, a pair of Clarki Colwns & and a purple dottyback. It is a mixed reef that I dose via dosing pump ESV 2-part & mag. I dose Micro bac7 once a week per the directions (~1.5 cap full), I use HW marinemix reefer. I do a 10 gal water (RO/DI 0 TDS) change every 2 weeks (I used to do 10 gal each week, but thought I would try every 2 weeks). I run a Pentair Smart UV 18W wi a flow rate of ~64 gal/hr (which I see is too slow for algea/bacteria). Ph ~ 7.8, Alk ~ 8.8, Calc ~430, SG ~ 1.026, Temp ~78.5, Phos <0.2, mag ~1500, NO3 <.1. I also run BRS GFO & carbon.

Sorry for the lack of phot0 skills!

Thanks,
Steve

 

[Randy Holmes-Farley Verified Community Expert]

Sand hardening is one way that calcium carbonate precipitation appears to show up, but every tank has some precipitation (like on heaters and pumps), but it often goes unnoticed.

It is not unusual for sand to harden, especially when it is new. here's my standard comment on it:

Clumping of sand may be purely abiotic, but may also be driven by biological processes, and most often happens with new sand.
Reduced precipitation can be reached with lower pH, alkalinity and to a smaller extent, calcium, and higher magnesium, organics, and phosphate.

Fresh CaCO3 surfaces can be most prone to more precipitation. To break the cycle, it can be useful to stop dosing for a few days, let alk fall and precipitation stop, then restart with a much lower dose, maybe using a lower pH product.

 

[bsn_rn_cen]

Thanks Randy often wondered about this myself

 

[flyboysmb]

Thanks for the reply Randy. My tank is not new, however, I guess any "new" sand I add would fit the bill as being new. Can you give me an idea of Ph & Alk levels that you would suggest lowering to.

Thanks,
Steve

 

[Randy Holmes-Farley Verified Community Expert]

Thanks for the reply Randy. My tank is not new, however, I guess any "new" sand I add would fit the bill as being new. Can you give me an idea of Ph & Alk levels that you would suggest lowering to.

Thanks,
Steve

In addition to the things above, stopping alk dosing for several to let the alk fall on its own to, say 6.5-7 dKH (or higher) is a good plan to stop excessive precipitation and allow time for the surfaces to get coated with other things.

As to pH, just stop doing anything to raise it (if you are) like a CO2 scrubber, outside air to a skimmer, etc.

FWIW, the precipitation process may already have stopped in your case. Its hard to know except by apparent excessive demand for alk and calcium, and it is often hard to estimate the "real" demand.