Any biomedia for a wet/dry filter is going to create nitrates. Biomedia just supplies a pourous surface for aerobic nitrifing bacteria to flourish, wether it be bioballs, biobale, biowheels, etc. Nitrates are the by product of the nitrogen cycle. Ammonia is broken down into nitrite, nitrite is broken down into nitrate via aerobic bacteria. If you have some nitrates, you know your filter is working. If you have excessively high nitrates it could indicate overfeeding or an undersized filter. Most invertabrates as well as fish will not be affected by nitrates unless they are present in large amounts, however ammonia and nitrite are toxic. If you would like to remove nitrates, regular water changes are the most practical solution. You can also build a plenum in order to host anaerobic nitrifying bacteria which will break down nitrates into a gas (sorry, don't know what type of gas); other than that there are some expensive filtration systems to remove nitrates. I'm not assuming you don't know this, however, just wanted to give you a thorough response just in case you didn't already have any of this info.