fw sand into sw tank?
 

buddy has a 220 g discus planted tank and wants to set it up to a fowlr and was wondering if he could just clean his sand really well and use it again he has 250 lbs or so
if he can clean it would vinegar or bleach work with a good rinsing after?? if so what concentration?

I would really really doubt it because the type of sand suitable for discus (neutral) is absolutely not suitable or not the best for saltwater fish.

He probably have silica sand? or maybe something else neutral? it's got to be because the discus need really soft water. The saltwater fish need a good buffer sand, something that can keep the PH high enough and contribute to the water chimistry.

I would simply discard, or rinse and sell that old sand and buy a good arogonite sand made for saltwater.

It is fine white saltwater sand it was purchased at hidden reef just before the doors shut..... His tap water was very soft so the discus were fine