How can you say you are sure it is cycled if you had just stated two factual measurements that say it most obviously isn't?
If there is a measurable ammonia+ammonium in the water by a liquid API test kit, your tank is NOT cycled and all your fish will get sick and perish, some sooner, some later, but there will be no exceptions.
Ammonia and nitrite MUST read zero ppm at all times. This is fish keeping rule #1. There is none more basic than this. Nothing else anyone does for their fish matters if this is not accomplished. As an example, you can place your dog or cat in the most exuberantly luxurious quarters and feed them the most expensive diet and hire them a personal massage therapist and get them a TV channel and the best toys and take them for long walks, but if the air they breath at home is contaminated with smoke, carbon monoxide, paint thinner, whatever, none of it would matter.
IDK what cycle by Fluval is but I hope you really got the ammonia detoxified.
I still would not recommend feeding until your tank's cycled. At the face value of your statements, you either don't understand what being cycled means or you are in some deep denial.
For instance:
The tank has been up for months I’m pretty sure it’s cycled
I have a bunch of other fish that are living perfectly fine in the tank it’s just one red tail that seems weirdd
You can have a tank run for a millennia but if the biofiltration is not coping with the stock, the tank is NOT cycled.
But you have never been forthcoming with the information much, so I personally find you hard to help. Yet, we will keep trying our best, bro.