Might be something dead in the pot, under the anemone..I dunnoI’m not sure you’re learning the lesson that TJ is trying to help you with. Or perhaps it’s just not coming across in your posts. Sorry if I come across as harsh. Ammonia is very toxic at low doses. You should not have any ammonia in a healthy reef tank. It should be zero. The fact that you have 0.25 ammonia after a 50% change is still significant cause for concern. Your remaining fish may die. If there are animals that have died in your tank, as they decay, they will break down into ammonia. Your relatively new tank biofilter might not be able to handle it, so your ammonia becomes detectable. If I had ammonia in my tank it would be a huge deal.
While some sort of infection may have started this (who knows) your immediate problem is the ammonia.
It’s really hard to give advice in this situation. It seems strange to be thinking about the flow or the nem or the flower pot with ammonia present in the tank. If there is anything dead in the tank, try to find it and remove it. Stop any feeding of the tank for now. You can dose prime into the main display. You can catch the fish and take them out of the tank into a QT situation with a heater and bubbler and maybe some live rock.