Turning a freshwater tank into a saltwater one uses basically the same technique as starting a saltwater tank from scratch. However, you benefit from not having to replace all your equipment. After finding homes for your freshwater fish, prepare to wait about a month to add new tropical fish to your saltwater aquarium.
The good news about switching water types is that you don't need a whole new tank. After dumping your freshwater, the substrate and decorations, clean the tank thoroughly with a diluted bleach solution, such as 9 parts water and 1 part bleach, and rinse well. Rinse other items you plan to reuse, such as your filter and thermometer. Don't worry about any bacteria that might still exist on these items -- they'll die out in the new saltwater environment.
The New Stuff
Although some of the big items from your freshwater tank are reusable, you must replace a few things, starting with the substrate. Freshwater gravel won't work well in saltwater aquariums, so choose one that helps saltwater bacteria grow -- such as sand, crushed coral or aragonite. Adding live rock gives you a base for living decorations such as coral while providing helpful bacteria instantly in your tank. You'll also need a few additional tools, such as a hydrometer to monitor your salt levels and potentially additional filters and pumps, depending on the number of fish you plan to keep.
Getting the Salt Right
Never use table salt in your aquarium; always buy a commercial salt designed specifically for fishy environments. These salt containers provide proper mixing information so you know exactly how much to mix with fresh, dechlorinated water before adding it to your tank. You can also buy ready-to-use saltwater aquarium water if you prefer. Even after following the manufacturer's instructions, you might need to add more salt or fresh water to the tank to bring your salinity levels up to par. In most cases, the specific gravity, which is measured by your hydrometer, should fall between 1.022 and 1.024. Some saltwater fish and invertebrate species require different salinity levels, but the basic levels should work for most creatures you add initially. When you perform water changes, check the salinity every time -- evaporation often means you must add a bit more fresh water.
Ready for Fish
After adding your substrate, live rock and saltwater, you still must use patience before adding fish. Just like with freshwater aquariums, saltwater tanks need time to cycle, which means giving the bacteria time to create a stable environment in the tank. Test for ammonia every few days, and you'll likely see it spike then begin to shrink. Don't add fish until no ammonia registers, which could take up to four weeks. Some fish, such as damsel fish, survive better in new saltwater tanks than others, so add about two of these fish per week for the first few weeks. Then, start adding other saltwater fish.