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For a 20 gallon, you could also try floating plants like Frogbit. They're great for shading and guppies love the roots. Just thin them out occasionally so they don't block all the light.

Vallisneria is another good option. It grows tall and provides cover for fry. Guppies don't bother it much, and it's hardy in a range of water conditions. Just make sure your substrate isn't too coarse.

I agree with flintech, your coral is a goniopora.