first filter change

Ron Hansen wrote:

You should be changing 10% (3 gallons) of water every week. Nitrates
are the end product of the Nitrification cycle that is possible in most
aquariums. Food rots and fish poop and pee and this produces ammonia
(toxic to the fish). Bacteria break down the ammonia, but give off
Nitrite (more toxic to the fish). Other bacteria break down the Nitrite
and give off Nitrate (much less toxic to the fish). No process in a
typical tank will consume the Nitrate. Therefore, you need to get rid
of it. By changing 10% of the water each week, you will dilute the
Nitrate and reduce the levels. If you don't, your fish will eventually
die.

There is one such process: Plant growth. Plants need nitrogen and will
absorb it either as ammonia or nitrate. That doesn't mean one should
skip water changes in a planted aquarium though, as fishes produce other
waste products besides nitrogen ('organic dissolved carbon'). But plants
can certainly help keeping nitrogen compounds at bay.