I keep a lagoonal tank with seagrass and other sandbed plants (Udotea, Penicillus, Halimeda). Remember that these plants need relatively intense, broad spectra illumination, like EYE 6500K. They are not as photoadaptive as coral. So the coral you keep with them need to be able to adapt to the type of light that the seagrass demand, or be positioned to only recieve the light indirectly. The coral should be lagoonal, not need extreme flushing currents (that would blow your sandbed everywhere), and be able to reject fine sediment.
I assume that you are going to emulate something like a patch reef abuting a grass bed. IME Favia, Favites, and Hydnophora are ideal for this type setup. Montipora that I've tried have all done well. Tank-propagated varieties of Porites, Pocillopora, Stylophora, Seriatropora, and most staghorn Acropora that I've tried do well but need relatively strong and random currents. Cycloseris - and I'd assume most Fungids - do well but need to be in areas of weaker current and sometimes weaker illumination. Tube anemones are easy but eat up a large radius (about 6") of tank space. I had a Macrodactyla doorensis host anemone, it's a denison of the seagrass bed, but it has an enormous tentacle sweep so it will take up a lot of tank footprint. The main problem I had with a host anemone was that it's hosted clownfish pair fanned up quarts of sand every day. All that sediment was not compatable with my stony coral so I eventually got rid of it.
T. maxima and T. squamosa have uniformly done well for me wether attached or on the bed. T. derasa would be ideal for a sandbed, I guess, but that clam becomes too large for my tank.
None of the Trachyphilia, Lobophylia, and Cataphylia that I've seen pass through the LFS during the last 3 years are ideal for a shallow set-up. They appear collected from deep water. Before my time in this hobby, I think Elegance were collected from shallow water, but the ones I've seen have the type of cone described as typical of deep-water ones. I do keep a red Lobo, but it only expands well if located under a rock, out of the strong light. The flourescent red and green "open" brains are not typical of shallow water, from what I understand.
I'd recommend Florida Pets for seagrass (they carry thee types), mermaids fan (Udotea), and shaving brush (Penicilius). They rinse the substrate from the seagrass roots
, but otherwise, IME sell a good product in decent shape. If you have Diadema or "tuxedo" urchins, ditch them, they'll reduce these plants to stubble overnight. You'll need a filthy, phosphate-rich DSB to support them, as they rely on the high nutrient level in the sandbed porewater to grow. On the other hand, the tank water needs to be lean enough to discourage the growth of filamentous algae which will choke them. IMO, regular additions of chelated iron helps most plants. Keep the carbonate alkalinity up to at least natural levels, I use limewater to bring in a balanced ratio of calcium when doing this. Other than that, I don't recommend any other additives. Articles on keeping seagrass in Aquaria are not common in the hobby. If you can get your hands on Adey's "Dynamic Aquaria", its has some applicable sections. I hope this was helpful, good luck.
