Join Date: Jun 2011
Location: Lake Havasu, Arizona
Well, you don't have to take out all of the water. The bacteria grows on the gravel and plants, so if you give that an inch or two of water, it should be good for any major changes you might have to do.
With 3 bettas you'll need to change out some water weekly to keep ammonia down. A filter will help out with a small amount of ammonia and so will your plants, but they won't take care of everything.
I'd do at least a 50% weekly.