Bettas need a MINIMUM of a 5 gallon tank (that will be a tank measuring 18x10x10 inches), heater, filter, and plants/ornaments. Fact. Anything less than that in my book constittues as neglect. Fish don't excrete waste only once every day. They don't breathe once a day. Their food doesn't break down into ammonia only once a day. They're doing all of these constantly. Without a filter, ammonia is free to build up between water changes. Fact. ANY detectable amount of ammonia is potentially dangerous to fish. Fact.
Just because your bettas aren't used to having a filter, doesn't give you an excuse to keep them without. Turn the filter flow towards the back panel of the tank to diffuse the flow.
Puka has provided you with links to DECENT advice from real, genuine fishkeepers. Take note or your fish will suffer long term.
There is not a fish out there that should be kept without a filter. To do so is negligent.