the water your using for the changes may have nitrates in it already, i had that problem with nitrItes, it was in my water supply and it took me a while to figure it out.
i dont think bioballs should get any more dirty then LR
if your water passes though a filter.
my bioballs are not wet dry, they just sit in a section of my supt where the return line is, the majority of my bio filtration i think comes from the ceramaic rings inside of a filter chamber.
a good use for a bioball i have found is get a 1gallon water jug make sure its not from milk, cut the bottom off and shove it upside down into your sump for your water inline, get a bioball and it fits perfectly on the inside where the water will come out then layer filter media on top of this, cut out some filter media that is larger that you need and place this peice on the bottom so that it goes up the sides, then stack filter media w/in that piece and have your return water income go through this filter.
i just cleaned mine last night after a week, it had collected alot of the sand dust from when added more sand over the past few days.
if you dont have sump, consider getting one, they keep alot of the unsightly hoses out of the main tank, especially if you have a small tank, and it allows for a small fuge.