Water changes
Image from Wikimedia Commons
Water change
The more often you change the water in the aquarium, the better it is. The ultimate is a system where water is continuously replaced, but this will be too complicated or expensive for most people.
A good rule of thumb is to change a third of the water in the aquarium every week.
Changing too much water at once, too often, can have a bad effect on the aquarium. If you change 80% of the water every day, you are probably disturbing the fish.
If there is a lot of chlorine (wp) in your tap water, you should consider temporarily storing the water for a day before putting it in the aquarium. Chlorine irritates the fish's skin and gills.
Sludge removal
When you remove water from the aquarium, it is a good idea to remove bottom sludge at the same time. This is done by attaching a so-called gravel vacuum to the end of the siphon hose that you use to get the water out.
Move the gravel vacuum over the bottom and gently into the substrate to loosen the waste. You can buy a gravel vacuum at the aquarium shop, or you can make it yourself for a tenth of the price.
Do not vacuum the substrate on the same day as cleaning the filter. Otherwise you risk removing so many beneficial bacteria that the aquarium has to cycle again.
Keeping a betta in a bowl
If you keep the betta in a "goldfish bowl", you should change at least a third of the water, 2-3 times a week or every day. In a bowl without a filter, there is not much basis for bacteria anyway, so there is little point in trying to take care of them. Make sure that the aquarium temperature does not fluctuate too much when you add new water.