No chemical additives should be added, they just pollute the tank in my opinion. Your water changes and lighting will keep your corals alive. I would wait about 2 or 3 months after the fish are added to try corals, they need a more established tank, some more than others. Anemones for instance need the tank to be about a year old for stability reasons. What kind of lighting do you have?? Snails and crabs can be added once the cycle is finished..I'd add them first before any fish.