RichardB wrote:

I have heard that with "fake rock" water features that are sold here (New
Zealand) they recommend if you want an aged, weathered look to brush it
with plain Yoghurt, it promotes the algae growth.....

There's a recipe somewhere for moss, that suggests mixing some moss with
yoghurt and covering the rock with it. I imagine it would work with algae. Derek

Since the kind of algae the OP is looking for lives under water, I
would think the fish would LOVE algae mixed yogurt, and wouldn't
last long. ;-) ~ jan
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Zone 7a, SE Washington State
Ponds: www.jjspond.us