I am a guy converted Africa cichlid tank to FOWLR saltwater tank 15 years ago. I can say what happen if he does that, I had spent too much extra money and too many times to drive here or there to buy some used equipment to upgrade my fresh tank and then buy the HOB skimmer and searching skimmer in the sump is better and cheaper, then setup the sump with diy pvc overflow. A month later, I realize the DIY PVC overflow won't work well and easy to get flood, then buy overflow box and go on and go on...
For cichlid fish, I had used lime rocks it is not the right rocks for saltwater fish, saltwater rocks is one of a filter for the tank and the light is also different.
One year later, I knew it is totally different. If I sell my freshwater tank include everything in there then buying the saltwater tank system from other hobbies in this forum is way way cheaper than buy separate.
My suggestion is selling everything include freshwater tank and take that money to looking for saltwater tank system (overflow build in) for less worry for flood, sand, rocks, sump, skimmer, pump return, all pipe setup, lights and may be also fishes may include in the system for cheap in this forum. He may add a little more money, but it is still the best way to go.
Another word is don't go around and around for a while then reach to point B, you can go straight to point B without any pain and headache.
Trust me, I am there before.
