Don't worry bro... I think your A N is over. Now focus on how to bring down the Nitrates. 50ppm is considered pretty high... and it's a sign that your Nitrifying Bacterias have not colonize the LR and filter media fully. In a healthy setup, the N.Bact should be able to break down the Nitrates... so eventually you will reach zero.
Some rely on external Denitrator (aka Nitratreductor) to allow more N.Bact growth. Adding N.Bact into the tank regularly is a good practice too... they come in tablet and liquid forms.
Water changes help too, but right now... just let your tank cycle for a bit longer before stocking. My new tank NO3 is around 15ppm and I am already panicking right now. I might get a Nitrareductor to help the situation.
There's another product called AZno3 that claims to eliminate NO3. Have not use it myself.
Hope this helps