What should be put under the cement ornamental fish pond?

I don't know how deep your pool is. You can get some mud at the bottom of the pond, or get some soil if you can't find it. Then plant some water lilies and aquatic plants, and spread some pebbles on the mud layer, but be careful not to spread them too closely. If this is done for a long time, the water will form its own ecosystem, and the sediment in the pond will decompose the excrement of fish and form nutrients for water lilies. Aquatic plants can release oxygen for fish to breathe, and you can also put some snails to remove excessive algae in the water.