Crushed Coral in a 20 long

[Leo2o915]

How much should I use to keep my ph stable after using RO ? I ordered a Pound from Aquarium Co Op want to use it in my filter or is it better under the sand in the substrate 

[KBOzzie59]

In my observations it's best to be in a place where water can move past it, that's why it is great in a filter.  If you put it under sand it would in essence be "capped".  The 3 methods I like.

1. In a media bag hanging in the tank near a filter outlet or outlet of a powerhead.  Can be a bit unsightly in a display tank.

2. Physically in a filter, HOB, canister, etc.  Out of sight, generally with good flow across it.

3. Physically as the substrate.

As for how much, I suppose you could use to little but it really depends on your options for getting it in the tank.