DSR Fe+ is a Iron based Liquid Phosphate remover
It removes PO4 by precipitation with Iron which get skimmed off and it's also being filtered by the power filter ( pump with filterwool used in DSR). Providing the reef had enough streaming pump it will not settle in your gravel any more  or less  that any dirt particle.

Preparing the Calcium stock solution:

• Weigh 700 grams of DSR Ca+ (CaCl2) and top up with reverse osmosis water to the 1-liter mark. (Use 696 ml of RO water)
• Stir or shake until everything is dissolved. The liquid will turn white and become hot!
• Let it sit, and the liquid will clear up.
• If necessary, top up with water to the 1-liter mark.
• Ready to use!

It’s an ALL-IN -ONE maintenance solution for marine aquariums. It is particularly suitable for both inexperienced and advanced aquarists looking for a way to successfully maintain their aquarium with minimal effort.