Looks like your ammonia is at .50 after the water change??? I would say your ammonia was probably around 1 ppm
before you did...say...a 50% water change. That would leave your ammonia at .50. I think you should probably do a couple of large water changes without a gravel vac if you've already done it. Allow around 6 hours between the changes. Test the water tomorrow and see where it's at.