Should be in the plant section but not an issue.
All sword plants (Echinodorus species) can grow in sand or gravel. They like nutrient rich substrate and I found they did best in pots/ plastic containers with at least 3 inches of gravel or sand. I prefer gravel but they grow in either. You can also grow Echinodorus in potting mix in pots, or just in the garden. I have one that is in a pot in my loungeroom. It gets treated like a normal garden plant.
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We use to grow some plants (usually swords, crypts, Aponogetons and water lilies) in 1 or 2 litre plastic icecream containers. You put an inch of gravel in the bottom of the container, then spread a thin layer of granulated garden fertiliser over the gravel. Put a 1/4inch (6mm) thick layer of red/ orange clay over the fertiliser. Dry the clay first and crush it into a powder. Then cover that with more gravel.
You put the plants in the gravel and as they grow, their roots hit the clay and fertiliser and they take off and go nuts. The clay stops the fertiliser leaching into the water.
You can smear silicon on the outside of the buckets and stick gravel or sand to them so it is less conspicuous. Or you can let algae grow on them and the containers turn green.