how to lower nitrates

my tank has been set up for 2 1/2 months now i have 2.5 inch sand bed 20 puonds base rock hob filter 2 damsels 1 med chacolat chip star 2 nassarius snails 5 hermit crabs 1 oyster i guess it came on a small piece of live rock and moves around the tank and 1 feather duster all in creaters seem to be doing great even when i had high nitrate leves the problem is i want to add 2 corals and cant get my nitrates down good enough i started with 35-40ppm and in 1 week did two 10 gal change in a 30 gal tank only about 25 gal water in it witch put me down to about 20 ppm waited 4 days and did 1 20gal change and my test kit still says i have some were between 10 and 15 ppm could somebody please help me out with some ideas of nnr or a way to keep it from going up so fast i have already bought the corals a open brian 4inby 6in and a tonga mushroom 2in by 4in i have to pick them up on the 14th and want them to be ok any help would greatly be apprieciatde thanks

 

You have not given enough information to respond on the addition of animals at this early stage, however, the nitrates are doing what they are suppose to do. they are comming down but will take some time to lower enough and stay low as long as you take out more than you create by feeding, fish poop, ect. you system is not seasoned well enough for corals in my opinion, and you should be able to keep water parameters stable and acceptable for at least a couple months before adding any more animals. I wish I could give you some good advice but at this time with a 2 1/2 month old system with live stock already in, there is not much I can share with you except slow down and be patient. maybe the lfs could hold the corals for you a bit longer to give your system a chance to mature a bit more. anyways if you do add the corals, good luck and test for ammonia, nitrite, nitrate, calcium, alkalinity, ph, temperature and assure all are stable and in the acceptable range. Keep us posted on your progress.