Looking at the pH test kit, it shows the wide range pH, which is of little or no value except to tell you whether you need the standard pH test kit or the high range test kit. Typically we would be using the high range test kit as it has much closer readability. Even more important a test is the KH test, which will indicate the pH stability over the day night changes.
The symptoms that you first described sounded like normal birth defects, but with the change in the habits of the fish, there is something that is stressing them out. It could be the salt, it could be something with the water. I would go back to using the Safe and treat for the entire tank volume. It will not cause problems and will prevent problems with ammonia, which will be present in your tap water if your water is treated with chloramines and will also take care of any differences in the level of chlorine added. Get a KH test kit and assure yourself of a minimum KH of 100ppm (6 drops) by adding baking soda if needed to increase to that level. Also get the high range pH test kit. pH will be somewhere around 8 to 8.3.
I am not familiar with the antibiotics chosen, so I don't know what side effects you should watch for.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard