The following information is taken from a much larger article on this blog and separated like this for ease of finding it. The full article and sources, which cover water quality and the Nitrogen cycle, can be found here.
Water Change Schedules for Best Tank Health
A clean source of water is an assurance of better betta
health. However, water changes are dependant on tank size. This next
section is a basic guide to the most popular tank sizes used for betta
fish. Bigger is always better and means less changes, especially if
1 gallon: Do a 100% water change every other day. This sized tank cannot establish a water cycle as it is too small.
50% water changes 2-3 times a week and one 100% water change weekly.
Another tank size too small for easy cycling, it is important to maintain
regular water changes. Gravel vacuums will make cleaning easier and help maintain better water.
This is the first tank size that can be cycled by beginners and thus is has two
different water change schedules. For cycled tanks, perform 50% water
changes weekly. For non-cycled tanks perform 50% water changes twice a
week and one 100% water change weekly. Non-cycled tanks are much harder
to clean and it is recommended that a cycle is established for the this
10 gallons: For
cycled tanks, perform 50% water changes weekly if there is only one
betta inhabiting it. If it is a community tank, add another 50% water
change to its schedule. For non-cycled tanks 50% water chanes should be
performed twice a week and a 100% water change bi-weekly.
20 gallons: Tanks this size should only be cycled. 50% water changed should occur each week.
are many other tank sizes but these are some of the most common. For
in-between sizes, adjust the schedule to your best judgement. Although
these are not absolute, strict, must-have schedules, they are
recommended for the best health of betta fish.