To convert;
Saltwater tanks are a lot more complex than FW. You need to do alot of reading. Empty all the water. It is no good, FW bacteria ( for short ), chemicals, and heavy metals. Fill it up with pure RO/DI water. You can't use tap water for a SW tank. Add a synthetic salt made for this purpos, obtain a specific gravity of 1.020. Ditch the filter. It possesses bacteria not beneficial to SW, It will compeat with the stuff in the LR. Add LR. Let the tank cycle. Wait until ammonia, nitrite, then nitrate to spike, then fall to 0. At this point you can start W/Cs, add a CUC and get a skimmer. Don't cheap out on skimmers, it is you life support, you get what you pay for. By this time ( a few months into it - you must take it slow ) You should have gained alot of knowledge. You can consider fish to add ( you can only have one, two at the most ), a damsel, a pair of small clowns, or a couple goby's. If you want to go reef extra preparations are necessary. Believe me, you'll enjoy it.