It depends on the size of the tank and what else you may have in it. If you have a very small tank (2 gallons or less in my opinion), I recommend that you change 25% of the water twice a week, and replace with conditioned water. This can either be tap water that has been left out for a day in order for the chemicals and minerals to evaporate, tap water with conditioners added (Quickstart is one I use), or the bottled betta water found at many pet stores. Having a 20 gallon and a 5 gallon tank, I add conditioners to tap water for both. Put your betta in a small container with the tank’s old water, then do any of the above.
Next, clean the algae and other stains. When I kept bettas in small tanks, I would wipe the inside of the tank down using a paper towel. Rub it all out and do not use any chemicals to do this-they can kill your fish. If you have a larger tank with filters and maybe other critters to help clean the algae, you may only need to do this occasionally (I recently cleaned some algae off the walls of my 20 gallon for the first time in recent memory, and it’s equipped with a filter, a siamese algae eater, and 2 otocinclus catfish). If you have synthetic plants in your tank, they will need to be cleaned regularly, as they get dirty very quickly.
With a small tank, after letting the water sit for conditioning and temperature adjustment, add the new water and then the betta.
This is my routine for keeping a betta (and numerous other animals) in a 20 gallon tank, plus 3 glo tetras in a 5 gallon tank:
- Vacuum the gravel using a siphon. I take out 25% of the water once a week in both tanks. This removes old food, fish poop, and other particles.
- Condition the water, using Quickstart, Aqueon Conditioner, and Seachem Flourish XL (this last one is a carbon boost for the live plants in my 20 gallon). Let sit to adjust for temperature
- Wipe algae. I do this every week for the 5 gallon and rarely for the 20 gallon.