Why are you adding bacteria?
The gill cover doesn't normally grow back. It shouldn't affect the fish as long as it doesn't get infected over the next few days. If you do a water change every day for the next week it should allow the gill cover to heal in cleaner water and reduce the chance of infection.
If it gets white fluffy stuff on it then it has a fungal infection and salt or Methylene Blue can treat it.
If it gets red ad inflamed it has a bacterial infection and salt or Methylene Blue should treat it.
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SALT
You can add rock salt (often sold as aquarium salt), swimming pool salt, or any non iodised salt (sodium chloride) to the aquarium at the dose rate of 1 heaped tablespoon per 20 litres (5 gallons) of water.
Keep the salt level like this for 1 week. If there's no improvement after a couple of days with salt, stop using it and use Methylene Blue.
The salt will not affect the beneficial filter bacteria, fish, plants, shrimp or snails.
After you use salt and the fish have recovered, you do a 10% water change each day for a week using only fresh water that has been dechlorinated. Then do a 20% water change each day for a week. Then you can do bigger water changes after that. This dilutes the salt out of the tank slowly so it doesn't harm the fish.
If you do water changes while using salt, you need to treat the new water with salt before adding it to the tank. This will keep the salt level stable in the tank and minimise stress on the fish.
When you first add salt, add the salt to a small bucket of tank water (2 litres or 1/2 gallon) and dissolve the salt. Then slowly pour the salt water into the tank near the filter outlet. Add the salt over a couple of minutes.
