I was hoping to get some help to see if anyone knows why my basil plants start out good and then slowly start having brown and black spots in them and look like they're being chewed off in random places and holes in the leaves. I've fertilized and sprayed with thuricide (BT) but no luck fixing the problem.
And I only fertilize like once every couple of weeks. Any help would be greatly appreciated!

I think you are over fertilizing. Plants need less fertilizer in pots and basil needs less fertilizer than most plants. If you fertilized every 6 to 8 weeks with half strength fertilizer, it would be plenty.

I agree with Daisy, to much fertilizer. When you get new plants I think you should use larger pots, basil can get really big and it gets a huge root system

I too agree, over fertilization. I actually grow basil with no fertilizer, and it does fine here in the northeast. I leave it alone in the ground in some good soil and let it grow. Some basil I have growing out front is lemon basil, and I used it as a ground cover around some plants. Comes back each year.
Have you thought of not letting it grow in a pot, or using a larger pot instead?