Oh, OK, So he was pretty big and mature when you got him, so he might indeed have had some age.
It normally takes about 2 months to prepare the bacteria before the tank is ready for fish. Since your tank is 3 months old that would mean that either the fish have only been in the tank for a month or that they were present in the tank while it was "cycling."
The reason we ask, in the beginners forum, about what test kit is being used and what the actual numbers are that the test results are giving is because we've found in actual practice that there is huge variation in what beginners are told in shops or get from friends or books and sometimes it is at significant variance with what our experienced fishkeepers have found necessary. Our aim is to help.
~~waterdrop~~