The fact that you have done water changes and the reading hasn't lowered tells me it probably isn't the tank...it just can't happen that nearly a 100% change in water in a short period of time doesn't bring the level down to much lower levels...it is the simple idea of dilution.
You said you got the kit at the same time as the tank. I would still take a tube to the LFS
and have them test it..see what they come up with.
You had indicated a couple of posts back that the inhabitants seem to be OK..is that still the case? Nitrates that high for this long would have done a lot of damage.