Jerry, you have two different readings for your nitrate reported above. I'm assuming that the first one, at 5.0, is supposed to be nitrIte. 5.0 mg/L (PPM) is actually the maximum on the most common charts, and is really a very high level. While nitrIte isn't 100% lethal to fish, it is extraordinarily stressful. In a healthy, mature aquarium, nitrIte (and ammonia) should never appear at more than a trace.
A higher level indicates a breakdown in the biological filtration of the aquarium. This can be for several reasons. The most common is that the biological filter has not yet fully matured -- I don't know how old the tank is, and whether this is the case.
It could also be from a die off hitting the filter, such as if it was hit with chlorinated water (very unlikely), antibiotics, or other chemicals.
Lastly, and most likely with a mature tank, it indicates a huge influx of something toxic -- someone dumped a can of fish food or a bologna sandwich in the tank, or a fish died behind a rock and you didn't remove it. It can also come from your source water, if -- for instance -- your tap water has chloramine rather than chlorine, and the prime dose was sufficient to break the bond between chlorine and ammonia, but not neutralise the ammonia. (Chloramine is a combined chlorine and ammonia dosage. It's used to sterlise our drinking water, and is more stable and effective than just chlorine. It's also more toxic to fish. When dosed with a sodium thiosulphate based dechlorinator, such as prime, the dechlor first breaks the chloramine into chlorine and ammonia, and neutralises the chlorine ion. This takes about twice the amount of dechlor as just chlorine does, but your "usual dose" is typically enough to do this anyhow. The ammonia is then free to wreak havoc in the tank. Most dechlorinators will also neutralise ammonia, but take a heavy dose to do it -- you want to use a double dose of prime if you've got chloramine).
It's also possible that your Prime has neutralised any ammonia, and it is testing as zero. Another huge dose of prime will help with the nitrite, too. Also keep in mind that if you are having an ammonia problem that is not being detected, if your brought your pH up with the water change, you increased the toxicity of the ammonia (and a big change in pH can really mess up discus anyhow).
Assuming this isn't a typo or some such, I think it's your culprit. I'd try to find your source of nitrite, and eliminate it. Its possible its the tap.