Instructions
Remove water from your saltwater aquarium. You have a few options for how you go about doing this. You can dip a plastic container into the water and lift the water out. Alternatively, you can use an aquarium siphon, which pulls water out of the tank.
Vacuum your tank's substrate. Most aquarium siphons come with a vacuum head you can guide across and into the substrate to agitate it and remove hidden waste products (uneaten food, fish excrement) which, if left untouched, will rot and cause problems with the tank's chemistry.
Add new water to your tank. You can't add just any water; it can't be water straight from your tap. Replacement water must match your aquarium's chemistry. You have to mix up replacement saltwater with the correct pH, temperature and specific gravity. This is accomplished by adding aquarium-grade salt to a mixing tub full of dechlorinated, heated water. Alternatively, many local fish stores will sell you premixed saltwater.
How to Reduce Nitrate Levels in Saltwater
It took a lot of work to set up your saltwater aquarium and you want it to thrive so you can enjoy your little slice of oceanic life. That's why you change your tank's water. The most basic care requirement for an aquarium is the water change: removing tank water and replacing it with new water. Water changes help control the chemical stability of your tank. In a properly cycled aquarium, where ammonia and nitrite levels are at zero and nitrate levels are as low as possible, regular water changes help remove nitrate buildup, which can cause excess algae growth and can harm any marine invertebrates you have.