Get us readings on your tap water, and a set or readings on your tank right now. Continue with twice daily water changes at 10%, using your conditioner, no pH buffers. Continue with the gravel vac at each water change.
Feed lightly, once a day, every other day. We need to get the tank conditions as close to your tap water as possible for future consistently, but this needs to be done slowly. Besides the possible onset or exsistence of OTS, you have this pH shifting by chemicals going on. If you lose all your fish, and you may, before you get this under control, other corrective things will come into play.
Please get those readings for us.