What they said. Make sure you cycle for at least a week or two. Unless you are like me who added fish on the second day but it was a 2" fish in a 75 long with 6x turnover rate
Also, your fish should have no problem finishing what you give them. If you feed them til they stop eating, then you feed them too much. Not trying to be a debbie downer haha, just saying it'll probably help your problem.
Bigger more frequent waterchanges. You'll need to do at least a 50% WC and refill with water containing 0 nitrates to get the tank's nitrate back to 20