Nitrates aren't an issue it's nitrites with an I, you have to check ammonia, NITRITE (more deadly than ammonia), nitrates are the final result if the nitrogen cycle and only way to keep them at bay is to do pwc's /live plants, but nitrates aren't deadly at even 100 ppm, now please don't get nitrates and nitrites mixed up ammonia should be 0, nitrites 0 then finally nitrates I don't even worry about anymore but 10-60ppm. Do a 25%-30% water change treat the new water with prime. Did you change cartridges in the filter? Anything new?
If you replaced the cartridge, I had mentioned to you a while back not to do this you throw away your beneficial bacteria, if the cartridge needs replaced you cut the material off the old one and stick it in the filter, or better yet modify the filter with media so when you do change a cartridge the media is still there with the beneficial bacteria in it, you can get media for like $2.50-3.50 with the mesh bag all you do is rinse the media off put it in the mesh bag, secure it shut with the clip and put it in the filter in front of where the water flows back out to the tank.
