I'm going to guess by you mentioning fluval that you have a canister filter? If so, I guarantee that's where your nitrates are coming from. When I first started my african tank, I was new to canisters. I didn't realize they needed to be cleaned out every other week. If you're not rinsing the media off and cleaning the sponges out, you're going to have nitrate issues. Don't replace the media or sponges, just rinse them out in a bucket of water you take out during a PWC.
Prime is a de-chlorinator. If you're using well water, my guess is you don't use a de-chlor. It definitely won't hurt, but it's not going to remove nitrates. It will take care of heavy metals, but that doesn't seem to be a concern at this point.
50% weekly PWC's are definitely a good idea. I do 50% in all of my tanks weekly. I have a 150g and a 40g, and am setting up a 125g. Sure it's a lot of water, but clean water is the absolute best thing possible for your fish, along with a good quality food like NLS.
Also, ditch the carbon. Unless you have a bad smell or are removing meds from the tank, it's not needed. It stops working after a week or so, and eventually just starts leeching whatever it absorbs right back in to the tank. A waste of money IMO.