Old sand will release PO4 with fluctuations in pH. And yes, we do keep our tanks acidic. At night my tank could be pH 8.4, but in the morning it could be pH 7.9. This is much more acidic than th enight before, and therefore caused some PO4 to leach into the tank. It may very well be re-adsorbed later in the day as the pH rises, but likely it will be availabel for algae as well. To remove PO4 entirely from rock, as we inferred for the cleaning of sand, an acid bath needs to be used to completely remove the outer carbonate layer of sand where the PO4 is laocated.
The problem is that sugar sand doesn't have much more left to it after the outer layer is gone. It just turns to muck.
Now if I add new sand to a tank laiden with PO4, sure, it will adsorb it. But if the tank is clean ond free from excess phosphate, it won't adsorb it at any apprecialble rate, and certainly I wouldn't refer to it as a "cycle" but rather a long process. You would expect this new sand to last several years, assuming you followed a strict maintanence routine for the rest of the tank.
__________________
Brad
|