1) Algae is really only an aesthetic concern, unless it grows so thick on the top surface of the medium that it clogs all air flow. In 15+ years of S/H growing, I have only seen that on a half-dozen pots.
2) If you don't like it, a periodic drench with a solution of Physan (1 tsp/gal), or liquid chlorine bleach (1/2 oz/gal) will keep it at bay.
I have used a product called Zero-Tol in the past. It is a stabilized, concentrated hydrogen peroxide product that is far more friendly to plants than the others. It is stabilized with perchloroacetic acid, so smells like vinegar.
Up until now, it was only available in large containers, and being an oxidizer, was a hazardous material for shipping. They now have it in ready-to-use, hose-end sprayers (I'll be getting some today), and ready-to-spray bottles (still seeking a wholesale source) that are shippable, so I'll be adding to my product line soon.
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