Can I immediately add some invertebrates to a new saltwater aquarium?
Your tank is too far into the cycle to add anything to it. Even with only two days your ammonia has started spiking. It will kill just about everything you could add to it now. There are also more humane ways of starting you cycle than adding something that will die. You could add one small uncooked shrimp, or some fish food. These things will decompose and produce the ammonia that you are after to kick start you cycle. also if you are interested in keeping invertebrates you should raise your specific gravity to around 1.025 and lower your temperature to at least 80F. also be aware that if you are using a hydrometer to measure you specific gravity, most are calibrated at 60F. So a reading of 1.021 at 80F would be 1.019, your would be just a little lower than this. When you have it close to where you want it, take a water sample to your local fish store and have them test if with a refractometer. They are not sensitive to temperature.
