re: Just got back into saltwater aquariums. got sick deal

Really don't need to replace them with anything. The refugium you plan on setting up, in conjunction with all the live rock in the system, will provide all the necessary means to convert ammonia to nitrite to nitrate. Those nitrates will then be used by the macroalgae growing in the refugium. When you harvest the algae, you are essentially taking those nitrates out of the system.

If you want to replace them with anything, I would suggest filling the area with small pieces of liverock.