if my salinity is 21 instead of 30, what does that do to my salt water fish?/?

Answer 1

They need to work harder to regulate the salt content in their bodies. Saltwater fish regulate the amount of salt they retain through a process called osmotic pressure. They drink a lot of water, and expell a lot of salty urine. If a fish in placed in water with a salinity different from what they are adapted to, they have to work harder, use up more energy on osmotic regulation, and their overall health will diminish, though they likely will not die as a result.

Add some water to perfect salinity.

Answer 2

This will cause your fish to have to work harder internally to keep their internal salt levels in check.

Due to osmosis the salt within their bodies is going to want to leave and enter the water.

The overall effect is that their immune systems will decline and make them far more susceptible to disease.