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I'll hazard a guess just to keep this ball rolling.

I'll propose that the higher oxygen levels in a tank where the plants are doing well acts to suppress algae. How? Um, maybe the algae is so small that it gets oxidized? That is, the oxygen molecules tear it apart?

Someone like TBarr will probably come along and say "No, I've tested tanks with algae and no algae and both had the same oxygen level." If, so, then that theory is kaput, but I'd be interested in knowing if the correlation was studied.

TW
That theory might get some support from the people who dose their unplanted fish tanks with hydrogen peroxide to "bleach" the algae. Once the algae turns white, it rarely returns. A higher oxygen level in a planted tank might just work the same way bleaching the chloroplasts out of the algae cells.
 

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It seems to me the answer is far simpler than most of the postulates in this thread. photosynthetic organisms need four things, basically:
water
nutrients(NPK-SFeMgCa-various micros)
a carbon source
light

In an aquarium we control all of these to some degree. Both EI and PPS are about creating a situation in which the light is the limiting factor. Any limiting factor other than light leads to the much faster adjusting algaes capitalizing on the excess light, and what is deficient determines what algaes will persist. Higher plants will monopolize the light so long as their other needs are met, leaving any algaes to starve.

The reason I rule out allelopathy is fairly simple. If the reason were allelopathy, then not only would the algae disappear but we would also see a single plant species completely overtaking the system. Occasionally this may happen, but for the most part planted aquariums grow multiple species in a single tank with only minor competition starvation issues. True allelopathy leads to single species dominance(like what is seen in sunflower fields, oak stands, eucalyptus stands, walnut stands....) This isn't the case in the majority of tanks, so allelopathic action is generally not the case.

Occam's razor is your friend.
It's usually our intervention (pruning, harvesting) that prevents a succession of dominant plants. I was just looking about week ago at a 120 gallon tank that had turned into a solid green block of Italian val.

Floaters and bryophytes (mosses, hornworts, crystalworts, etc.) are prone to taking over too. We tend to keep those out, or severely control and harvest those in a planted tank. I set up a planted tank at my daughters house. Basically it is now nothing but small duckweed on top and the rest solid with Chara cf. contraria. I suspect many of us skipped or forgot that early phase in aquatic plant keeping when we assumed that plants would all get along. She just now caught on and is starting to regularly pull out the excess Chara and duckweed.

I used to keep large tanks of marine plants, some true vascular plants, most macroalgae. I would start out with over forty different species of sea plants in a tank. No matter how much manipulation I tried, a handful of species or less would dominate and many would vanish or seem to for a while.
 
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