I am fairly new to the planted aquarium community, so please solve
some incongruities, if you could.

When I had just plastic plants in my aquarium, I was told to change
15%-20% of the water every week because of the build up of Nitrates in
the water. The idea, as I understood it, was that there is bacteria
that will take the ammonia produced by the fish, and make it into
Nitrite, and there was bacteria to take the Nitrite and turn it into
Nitrate, but that there was nothing that could really be done about
the Nitrate.

Now, I have seen that Nitrate is beneficial to plants, to the extent
that people seem to get fertilizers to *add* Nitrate to their
aquariums. Those same people seem to advocate changing their aquarium
water from 30%-50% (!) weekly/every other week.

It seems like a planted aquarium should not require very many water
changes, and almost no vacuuming (as fish waste is something that
plants like!). Am I mistaken, here? What is the reasoning, with a
planted aquarium, to change the water? Or is my understanding of the
chemical/bilogical processes involved mistaken?

Thanks!

-Harry