Well, my first response is sweetness for having that much room for your Betta!! Things to consider:
* What kind of gravel/flooring material do you have, and is it established or new? Do you vacuum the gravel or whatever you are using? Food and poop can get piled up in the "flooring" and cause ammonia problems. Have you made any kinds of changes? Do you use Aquasafe or something similar when making water changes?
* I recently had the same problem with my 10g, whitish cloudy for a couple of weeks. I did more frequent water changes (usually 2/week, moved to 4/week with lesser amounts). Used
Amquel--it's pretty good for dealing with Ammonia, nitrites, nitrates, etc.
* Are you using a tube test kit, which is much more accurate, or the 5in1 dip strips (not very reliable)? Test the water out of the tap for ammonia so you will have a comparison!
* Are you overfeeding him? They really don't need as much food as most people believe--that goes for most fish. I recommend the Hikari Gold if you aren't already using it.
* I've read that sometimes algae bloom can be whitish instead of greenish. Also, the aquarium might be constantly having to cycle since you're changing the water so often. Try twice a week, then once a week.
* For more experienced fishkeepers and to see this is a fairly common problem, go to Search at the top bar, type in "cloudy white water" and you'll find a whole list of threads on the topic!!!
*By the way, my fish were fine during this time--tetras, Gourami, and a mystery snail--no gasping for air, spending time at the top, or at the bottom with clamped fins, so your little mister should be fine.
I hope this was helpful; I was frustrated, too, when my aquarium did this. I had a male Betta over 3 years in a 2.5 gallon glass bowl but never had this problem with him. Good luck!!!