We set up our 90 gallon tank and put the live rock and sand and let it cycle. Once the ammonia and nitrite levels remained at zero for over a week we added a fire shrimp, about 10 small hermit crabs, about 10 snails and 2 clown fish. Everyone seems to be healthy and happy but now when I test the water the ammonia will not drop below .25 ppm and the nitrites are at a .5ppm. Nitrates are at 20ppm. We have done 20 gal water changes daily the last two weeks and those numbers will not change. What are we doing wrong? Any thoughts on how to get ammonia and nitrites back to zero?