I have grown the same mangroves in my tank for the last 4-5 yrs, brackish tank with salinity of 1.005 or so (range 1.003-1.007).
Most online and aquarium stores charge 5-10$ per pod/pland, which is outrageous. I got mine off ebay from a guy in hawaii who sells 12 seeds/pods for 16$ shipped. Just checked and he has sveral auctions ongoing.
There are several sites online describing germination using floating styrofoam with a hole cut in it to suspend the pod 1/2 way out ot the water. Slow process but neat. You just need enough head-room between your lights and the plant to prevent burning of the leaves. As they grow, snip off the leading meristem/growth point to "bonsaii-ify" them. You can also use rubber bands to pull down the stalk towards the trunk. I have several that have 5-6 "u-shaped" humps in the "trunks".
I have read that they do a good job of removing nitrates etc from the water, but I have my doubts. I grow them strictly for asthetic value. Someone told me once on this site (cdawson?) that they remove the magnesium from the water, so some dosing with epsom salt may promote healthier plants. I haven't done this but have noticed growth spurts after large water changes, so they do seem to remove some necessary nutrient from the water.