[Michael Hoaster]

Thanks 3D-Reef. That really is the key I think. How much nutrients to provide the seagrass at startup, to give them a head start without giving algae what it wants. It seems to me that putting the nutrients deep in the sand bed, while minimizing water column nutrients is a good solution. I agree that they will leach out into the water, but hopefully that will take some time, and give the seagrass a head start.

You said before, once you put something in the sandbed, it's there until you break down the tank. I get your point but on the other hand, I've added heavy nutrient input to my freshwater planted tank substrate, using plant tabs. The plants suck them up and grow like weeds, then slow down as the tabs get used up. I pushed them down about 5 inches deep, and I did not notice any uptick in algae production in the tank. I'm sure some leached out, but overall the strategy worked. My plants outcompeted the algae.

I realize of course these are different plants, but I know that plants will use nutrients that are available to them, even if they are adapted to nutrient-poor conditions. Diana Walstad did some experiments that proved this pretty convincingly. Her book "The Ecology of the Planted Aquarium" is excellent.

I do agree with you that putting too much of my aged eco-complete in there would be a mistake. I'm just going to have to (re)read all I can find about seagrass substrates in nature and come up with my own secret sauce. If I recall, some exist in nutrient-poor substrates and some in nutrient-rich ones.

I think our biggest difference of opinion is on the seagrass' ability to take up nutrients from the water column. Here's my point of view: these are higher plants with separate components that do different things. Basically, the roots take up nutrients and the leaves photosynthesize. They are not macroalgae, which are much simpler plants that do get most of their nutrients from the water column. So in my opinion, feeding the roots and not the bulk water is the best way to favor the seagrass.