This article is from the Aquaria: Plants FAQ, by multiple authors.
If you are adding that second light to your tank for the first time,
you should be prepared for this. Increased light is welcomed by both
algae and plants, so the plants must out-compete the algae. You can
help tip the balance in the plants' favor by maintaining a low fish
population, keeping algae eaters, and frequent water changes (see the
ALGAE SECTION of the DISEASE FAQ).
 
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