Need help!!

Hello everyone first time posting and I'm in need of some help! My tank is a 45g bf with 20g sump and about 60-40 mix between live rock and dry rock bout 75 lbs in total rock. I'm testing my water regularly and I notice before I was even half way threw my cycle that my nitrates were threw the roof... I let the ammonia and nitrite get to 0 but still left with a high level of nitrates... I do a water change it help temporarily... but I can't be doing a 30 g water change every day so really my question is where could these nitrates be coming from and how do I get rid of them efficiently and effectively. I'm running a pretty basic 150g protein skimmer and have deep sand beds in both my sump and dt .. live stock in the tank is about 5 hermits 1 peppermint shrimp 1 emerald crab 3 snails 2 clown fish bout 1inch in length.. my nitrates I bout 80 ppm ... the dry rock was in an old tank of mine could the rock be leaching nitrates from the matter that died on the rock once dry ... the tank has been up for 2 months... please help