Just set up a 72 gallon with a 20 gallon sump(refugium) set up. Gravity levels are good, currently have live rock and sand running though the sump. I want to know how long before I can put some fish in there?
Be aware it's expensive but it's worth it. The small bottle is only rated for 40 gallons. The next size is the 8 oz and it's recommended for 120 gallons.
Your tank will be ready when it can process ammonia and nitrite. was the live rock live? You should either dose plain ammonia or easier throw a cocktail shrimp in to let it degrade. When you see ammonia spike and then drop to 0 and then you see nitrite then you are ready to add some cleanup crew. After a little while longer you can add a fish. You are right now in the bacteria keeping phase and need to raise a colony that can support the fish. Don't rush it. Do some reading on the nitrogen cycle while you wait. Practice testing and water change routines.