Did you add a bunch of fish all at once to the newly cycled tank? If so, it's possible the BB was overwhelmed & you are experiencing cycling again or a mini cycle. What's the tank size? What fish are in it; type & #'s of each? What's the filtration? Have you changed out any filter media? When doing a water change with nitrites that high I would do a minimum of a 50% water change, wait an hour or so & do another 50% (minimum) water change. That should bring your nitrites down to 0 or at least VERY close to 0.
Sorry for the delayed response, I needed a day or two to do some water changes and get some more information. I will try to be as detailed as I can. I got the nitrite back to almost normal tolerances, but the amount of work to do so was obscene...
My tank is about 6 weeks old. Ammonia stays yellow 0-.25ppm lately, nitrites off the charts, and nitrates are always orange so 5-20ppm. 76 degrees, and pH is upper 7 range. I've slowly added a few different fish, most have been fine. Only deaths I've had was neon tetras, but I didn't realize they are NOT good candidates for tanks that aren't aged. Currently I'm a bit full, I'm returning 4 back to my LFS.
3 neon tetra
3 red-eye tetra
4 buenos aires tetra
1 pair lyretail mollies
1 baby pleco
Again, here is the picture from last Thursday, all nitrite readings. Vial #1 is tap water, vial #2 is tank water (1/3 WC each day for a few days prior to this reading), vial #3 is after a big 2/3 WC, and vial #4 is after an even bigger 3/4 WC.
I did a series of WC yesterday, Tuesday around noon. Vial #1 is tank water, vial #2 is after a 1/2 WC, vial #3 is after another 1/2 WC, and vial #4 is after yet another 1/2 WC. A 1/2 WC is about 8 gallons. I did the math on my tank when it was still empty, and it maxes out at 18.9g when filled to the lip. Given gravel, décor, fish, and the fact it doesn't get that full, I'm probably around 16-17g water.
Today, Wednesday morning, I did another series of WC, this time 9 gallon intervals instead of 8. Also thoroughly cleaned out the gravel to remove wastes. My Buenos aires tetras like to poop! Vial #1 is tank water, vial #2 is after 1/2+ WC, vial #3 is after another 1/2+ WC, and vial #4 is after yet another 1/2+ WC.
Everything is clean, there is no uneaten food, and whatever poop that was present is gone. I bumped up my aeration to ensure oxygen levels are fine. None of the fish "breathe heavy".
Today I discovered that the inside of my filter has an accumulation of scum in it, which wasn't there last week. I have a Tetra 30-60, which does a pretty good job. I haven't changed the media, but around week 2 or so I made a stupid move and rinsed out the media with hot water...