Pouring gallon jugs into what is basically a 4 block long shallow "lake" was probably ineffective (they would have needed a tanker of the stuff and since peroxide is unstable, it easily breaks down into water and oxygen anyway).
What has so far gotten rid of the most obvious algae, was their literal manual scraping and vacuuming the slime off the slab, section by section. That's what people do with fish tanks (I used to have 3 - freshwater tropical, saltwater marine, and saltwater reef with live sand and live corals). But in this case, that suctioning obviously loosened the coating and who knows what might be leaching into the water.
Bodies of water that are not maintained as highly-treated swimming pools, go through a natural biological cycle and right now, that has been disrupted. I saw some article where some CT-spewing MAGats were whining about someone putting phosphorus in the water. 
Who cares if there are large grassy areas on both side with plants and trees that are routinely fertilized with some N-P-K ratio of fertilizer.
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