If I read Tecohorn correctly, the pond has been unmanaged for 3-4 years. My guess is, that's not enough time for serious genetic selection of "small BG" genes to occur. But as ewest points out, if you can add some large adult CNBG they will help suppress maturation of existing small, juvenile BG. And adding some new BG genes to the mix can only help.

"Live like you'll die tomorrow, but manage your grass like you'll live forever."
-S. M. Stirling
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