The odds of you creating a true DSB with anaerobic bacteria in sufficient quantities is remote. For all the talk, I doubt there are many that maintain enough to process 20ppm of nitrate. I would look at how much you feed and the quality of the food. Even though there are no fish in there now, I assume you are feeding something. If not, the bacteria will die.. Anywhoooo back to where I was.. When you were feeding (let's assume over feeding , even a tad bit), all that excess is decomposing and resulting in nitrate creation. It's going to take time but the keys are a) water changes b) not to over feed c) quality food d) rinse alot (outside of the tank) those filters or get rid of them all together (better). If you have no fish and are changing that much water anyway, then why not do a 80%+ bu;k change. If immediately after that your nitrates still read as high (though as I said, 30 isn't THAT bad).. then it's either the kit, the operator or the water source