Some dry rock is high in phosphate. There are a number of threads that can explain the entire process better and in more detail, but I'll give it a shot.
I think for 150 pounds you might need two containers. Mix up saltwater with RODI to reef salinity. Add Either a piece of LR, Bacteria In a bottle, or nothing. Add a source of ammonia, to "feed" the bacteria, or start the nitrogen cycle. You could literally feed the barrels with pellets, or drop a raw table shrimp in the barrels.
Monitor ammonia, nitrite, nitrate and phosphate. As the weeks progress, you'll see the ammonia drop to zero and the nitrite rise, then the nitrite lower, eventually to zero, and nitrates rocket up. Once the ammonia and nitrite zero out, you might begin doing water changes to remove phosphate. Or while the nitrogen cycle is taking place, if the phosphate levels are high, you might hook up a phosban, or similar, reactor with GFO in it to help reduce phosphate while cycling the rock.
It usually takes 6-8 weeks IME, for the entire process to happen. Also depending on how low you'd like your phosphate level and if you're going to keep a reef/coral, or just Sharks/FOWLR.
HTH.
