Your tank, being only 3 weeks old, is overstocked, and attempting to cope with the bioload. Your fish are becoming stressed (thus susceptible to illness), most likely due to this. Do not be surprised if the ammonia peaks higher than it is already, since there is no nitrates per your description. Once your tank develops nitrites and nitrates, you know your on your way to being cycled.
SIDE NOTE: If you are using testing strips, can them, and go with a liquid test kit (Like an API Freshwater ammonia, pH, Nitrites, nitrates
at minimum). Strips will not give you the readings you need, due to being habitually inaccurate, giving false negatives/positives/readings.
If you want to correct this "naturally", attempt to correct the water conditions by small, more frequent partial water changes (2 - 3X per week instead of weekly), and
DO NOT ADD any more fish. Reduce the amount of feedings, which will help reduce the waste produced in the tank, which adds to the problem. Most of the problems in any tank/fish are a side affect of water conditions, brought on for whatever reason, and it doesn't take much to throw it off it seems.
If your tank was cycled to start with (a process that is always in flux depending on conditions of the tank), then the addition of all those fish are putting it BACK into a cycle again. My suspicion is that the cycle was not fully complete, then the overstocking of fish compounded the problem. Adding biospira may help, but ONLY if it has been refrigerated for the entire SHORT time you've had it. It is very subject to dying off if proper storage has not been followed, and after opening, must be used within a short period of time.
Stocking SLOWLY is the key in adding livestock to a tank, after the initial cycle.
Good luck with your fish. He/she may pull through, since it is still active and eating. You may have to isolate it if it shows signs of worsening.