There are different micro habitats in the rift lakes, open sandy areas, rocky shoals, and places where plants abound.
In many cases depth (where sunlight can reach) and substrate is the determiner whether plants will survive or not.
That aside, higher plants are not a problem for fish, especially in tanks because they tend to improve water quality.
Tanks like mine, that are heavily planted do have day /night pH swings though.
During the day plant give off oxygen, and use carbon dioxide in the presence of sunlight, helping to slightly raise pH.
But at night without sunlight, plants use oxygen, and give off carbon dioxide lowering pH (carbonic acids).
If I take an early morning pH reading of my tank it may read 7.8 pH, if I take another sample in late afternoon, that sample may show a pH of 8.0, or even 8,2.
Most algae serves a similar function, but.....there are certain types, like cyanobacteria and a few other pollution loving algal species that can have negative effects on higher forms of life like fish.
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My 180 gal in the photos above , and video below, nitrate level averages less than 5 ppm, it is stocked with 15 fish @ 5"( 12 cichlids, 2 tetras, 1 Pleco), and one 10" goby.
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Goby muggin camera