Sand should be fine, although alot of people have reported good results with gravel as well. Gravel allows for more oxygen to reach the roots, which is essential for any plant - but, as I said, sand works well too.
If you have a sump then you can also use them in there as well, providing you give them light. Don't sit a light too near the follage though as they can easily be scorched and overheat, although they are tropical and do need more than sunlight from a nearby window. The top half of the plant should be above water also.
They should be cut back every so often as well due to the fact that mangroves can grow to enormous proportions and are quite a sight in the wild (I had the aweinspiring chance of going out in a boat when I was on holiday in Darwin and seeing these majestic trees all along the coast and growing all along the banks of rivers that flowed into the sea, the abundance of wildlife was breathtaking as well, even saw a turtle putting alongside the boat). Perhaps borrowing a few books on general gardening from a library or running a few searches online can better enhance your green thumbs. Even learning a bit about Bonsai could be useful to you in regards to keeping these as well, as the same sort of pruning will be required over time to ensure your house doesn't become overrun by your plants lol
Don't blame me if you happen to get bitten by the garden gnome bug and contract a few new hobbies in the process though, God knows, I did
Between fish, Bonsai, Carnivorous plants and the missus, I barely have time to scratch my ass these days
