If the tank has only been running for 3 weeks it won't have fully cycled.
Sorry, I am not very technical and I'm sure someone here will provide a better explanation but cycling is the time it takes for the good bacteria to build up in a filter. When there are enough good ones they gobble up the bad ones!
Because it hasn't cycled, ammonia and nitrates will be building up in the water so you need to do small, frequent water changes. I overstocked a new tank once and did a water change of about 20% every night for a few weeks. You might need to add water ager (dechlorinator) if you water is heavily chlorinated.
Hope that helps.