Do not do 100% WCs. It can mess up the water parametersand stress out your betta. 50% is as much as I would do.
Well Known Member
In a bowl, depending on your filtration and your stocking, large water changes may be your best bet. 90% is a bit much weekly, but 75% is fine, and is definitely much better than having a betta living in filthy water.
If reef tanks can handle 80-100% WCs, your betta will be fine