You can use crushed coral if you like , I'm just not sure that would look very natural. I used peat for my background. Some use eco earth, coco fiber, and other various mixtures. What ever you decide to use make sure it is dry. You may also need 2 coats or more. Make sure each coat fully cures before adding the next. I would also let it finally cure for at least a couple weeks with the cooler temps.
I like to use pea gravel around my water features. Cut your screen long enough that a few inches drop to the bottom of the water area, then cover that with pea gravel making sure that your edges stay flat. You can use larger stones,drift wood, mopani wood or what ever else you may like. Here's a pic of mine with hardly any water on the bottom because I have a bunch of tiny froglets I don't want to drown. The Raider emblem is for everyone's viewing pleasure.
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