Need advice on keeping a Betta (fighting fish)

I currently have a Betta in a small tank with plants and all. Pretty bluish green body with a hint of purple in his fins. I find that the water gets dirty pretty fast and I have to change it once every 3 to 4 days. Will this stress out the fish? Im sure it does. Sometimes, I'm kindda cruel as I drop live ants I find occasionally crawling about his tank. But he seems to love it! I was thinking of transfering him into a bigger tank but I read somewhere that a filter will freak these fellows out. Is it a good idea to put a filter in the tank so that I will not have to traumatise him by changing the water so often? Much ado about a fish I know...but these fellows tend to grow on yea.. ;)