First, Check your Alkalinity and make sure it is at the correct levels.
If that is ok do a CO2 test... take a couple cups of water outside and use an airstone to airrate it for a couple hours then test it and if the ph tests higher, you have an elevated CO2 level in your house.
Did you use any concrete rocks in setting up your tank?
Just a few ideas let me know and we can go from here.
Brandon