old sand - use or toss?

Rinse it really well with the garden hose and you'll be fine, if your that paranoid about it just let it dry out after you rinse it. Then there wouldn't be anything left other than the aragonite sand. Either way good luck!

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It depends on the health of the system the sand came from. Phosphate binds with calcium carbonate. If the tank was dirty, there's a very good chance that the sand is loaded with phosphate. If you place it in a clean system, the sand will act as a reservoir for phosphate. This phosphate will slowly be released from the sand, fueling algae blooms and other problems. If you run phosphate reactors, keep the sand clean, harvest macro algae, and do regular water changes, in time, this phosphate can be brought down to manageable levels. If the system was relatively clean and healthy, there shouldn't be a huge amount of phosphate bound to the sand grains, and you may not notice any real problems by reusing it.

If you do use it, allow the sand to dry completely. Then place a portion of the sand in a pillow case, tie the end, and tumble the sand in the pillow case. This will cause the grains to grind against each other, knocking loose particles of detritus. This way, when you rinse the sand, it will be much easier to remove the detritus because it won't be stuck to the sand. This will also greatly reduce the amount of phosphate you are placing in the new tank.
HTH.
 
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