A ton of people say that mangroves should not be fully submerged. I added a few to a display tank for seahorses that are fully submerged and doing just fine. They are growing and seem to be as healthy as ever. If I were you I would completly submerge 1 or 2 of them and see what happens.
i dont remember seeing mangrove growing under water and i dont think they will thrive for that long been submerge if u guys see how they grow in the wild the are not under water just the root ... and i do have mangroves about 14 of them which are growing pretty nicely