I think you got your nitrite and nitrAte readings confused right? It's normal for nitrAtes to be around 20, but if your nitrItes were 20, well, your test kit wouldn't even read that What are your ammonia readings? Should be nil if you're seeing nitrAte, but always good to check, just in case.
Alsdo, did you buy test strips, or a dropper style test kit? The strips are notoriously inaccurate, so you may want to return them and get the dropper test kit if you can. I'd get the drop style kit anyways. The Aquarium Pharmaceuticals master test kit or Nutrafin mini master test kit are both good choices, and should be around 20 dollars. Here's a link:
Nutrafin test kit
Aquarium Pharm's test kit
I would try removing the pleco to a quarantine tank and salting the tank to 0.3%. Salt will take care of a lot of illnesses as you probably already know, and it's a good treatment to try as a first step.
0.3% equals 1 tbsp per gallon, added in 3 separate intervals so as not to shock the fish or the filter. So for a 30 gallon tank you would add 30 tbsp, divide into 3 10 tbsp doses, added 12 hours apart. Keep the salt in the water for 2 weeks, replacing what you remove with water changes. So if you do a 10% water change you've removed 3 gallons of water, which is 3 tbsp's of salt you'll need to add back in.
Plecos are salt sensitive, so you don't want to add the salt with him in there.
If they don't respond to the salt treatment you can consider other options, but what is wrong with them isn't obvious so salt is a good first step to take.