trackman said:would it help if i just replaced the cartridges instead of rinsing the out in fishy water
Yeah, dont do this. You want to preserve as much bacteria as possible.
Having nitrates in your tap water is undoubtedly contributing to your nitrate "problem". 40 is a little on the high side, but nothing really bad. I would do as Slappy said and vacuum under the rocks. If that doesn't help, you may want to consider changing the gravel over to sand.
-Diana