Tank has high Nitrite (NO2) levels and Betta Fish issue

Question
Our 14Gal tank has high levels of Nitrite(NO2)still. This tank started from a 5Gal tank (with male betta and albino catfish) that was running for approx 3 months before with no water problems (water levels tested fine with API test strips). Then the set-up was moved to the current 14G tank about a month & half ago. With water levels testing fine the first two weeks, we added some fish over weekly intervals. First was a small 3" plecostomus, then an 2" albino catfish, then 1 female betta (the male was removed after day two due to fighting - back to the 5Gal tank) and lastly a 4" plecostomus. Testing water levels were fine weekly until about a week ago. Then both Nitrite(NO2)& Nitrate(NO3)levels shot up (using the API test strips). The Petsmart staff suggested to do 50% water change, which I did. As of today, the nitrite level are still high (ran out of API test strips and now using the liquid test set).

What needs to be done to correct this? Is the tank just now starting to (re)cycle or is it over-loaded/adding fish to fast?

As for filtration, we have an Aqua Tech 10-20 and 5-15 filter (was in the 5Gal before) for the 14 Gal tank with two air bubbler lines. With weekly water chages about 10-20%.
Water level readings are:
pH   7.8 (should we be worried?)
Ammonia  0ppm
Nitrite(NO2)  5.0ppm+(off the scale)
Nitrate(NO3)  5.0ppm

The all fish appear healthy with no problems eating, swimming, or breathing/gills. Although, the female did get seem to get stressed from the male and was treated with API's Bettafix. Her front half of the body has not regained the blue color yet (still grayish). Is this something else?

Please point us in the right direction. Thankz.

Answer

Oops! I forgot to address the female betta issue...

She may still need time to recover from the fighting episodes. It's even possible she is ready to lay eggs and that's why her color is faded. Female bettas get a bit lighter and develop a series of vertical bars on their body to indicate breeding readiness.  

Chris

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Hi Mark and Tina;

Nitrite levels that high should be causing the fish to have trouble breathing. If the fish seem to act fine and don't appear to be stressed, I would have another test done with a different kit. The strips are actually not very reliable. The liquid drops kits are much better than strips and the tablet kits are the best, in my opinion. ;-)

At Your Service;
Chris Robbins