I would suggest waiting a few more weeks before adding another fish. After a new waste producing animal is added you need to allow the bacteria time to catch up to the new load. Bacteria grows to match the waste produced and is not "always abundant". If you are adding another percula, try getting one that is much smaller than the current one and you will have less issues with fighting.
In future when adding new fish to the tank, please place them into a quarantine tank
first. Keeping them in a QT for a period of 4 weeks prior to adding to the main tank will prevent many issues. It allows for proper observation to be sure the fish is free of parasites, viral infections and other problems that will afftect the life of the other inhabitants. If needs be the fish can be easily treated and if done properly, the main display tank should not have any future problems from outside sources.
It also allows sufficient time to pass between fish additions.