Is it possible to make my 20 gall in to a FOWLR

Ok so i was thinking since i will be keeping my puffer in his 20 gallon, is it possible for me to make it a FOWLR tank? I already have the s***** skimmer, filter, and heater. For subtrate i have black Tahitan moon sand, and a decoration that he loves. Would it be possible to turn his tank to a mini fowlr. I really don't want to have to take out the subtrate so what if i were to get a cup worth out of my 46 and put it into his tank *even though it would look bad* but it would at least start to get some life in there, and just buy some live rock? For water i use tap and mix the salt in the tank. The tank has gone through a freshwater, to bracksih, to high brackish to marine stage.

Would this be possible?

 

yes, just add tap water conditioner, if you are using tap water(Ro water is much better). Also, the life will grow on its own, no need to mix the sands.

 

Depending on the puffer, the tank might be too small though.

 

its a green spotted, oh and i also use prime as a water conditioner