Floating Plants with Mbuna?

My P. saulosi made quick work of a Riccia fluitans order. Gone in two days. I have tons of duckweed which they have not touched. I wish they would eat it.
 
I just put in what I like upon looks and heck if they eat it and tear it up o well plants are cheap and they like it :) the only thing they do that irritates me is get sand all over the leaves lol
 
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