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I would cap your gravel with an additional layer of sand. About an inch or so. You're having too many nutrients leak through the gravel into the water column. I have 5 Walstad tanks and whenever I had green water or algae blooms, I would add some sand and it stopped the problem immediately and the plants did better and the fish began to thrive. Gravel (especially if not deep enough to cover the soil) lets too many nutrients seep through too quickly whereas sand slows down the process enough so the bacteria can break it down. Too much nitrogen and phosphates from the soil feeds the algae. So add a sand cap and you'll be amazed.
Interesting results, but I would be very careful about making such a bold recommendation. I would want to know exactly what your substrate is, how old the tanks are, and see 'before and after' pictures of at least one representative tank. And I would start your own thread on this subject where it can be documented more carefully and then discussed. It seems a little out of place here.
 
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