nitrite level increased

Question
I just did a major change in my 10 gal freshwater tank- have had a few fish die recently (whirling disease in one molly, unsure about two others, removed a beta with large swelling and placed in hospital tank).  I completely emptied the tank, disinfected it with an oxidizing cleaner, thoroughly rinsed it, replaced all of the gravel, new artificial plant, same filter, new filter (no charcoal) pad, same heater.  I added copper safe to the tank, a neutralizer and Stability.  I checked the water today (3 days post change) with a strip test and everything is within ideal range except the nitrites (pad is brown).  Remaining fish (6 neon tetras, 2 rasboras, 2 chinese algae eaters) seem active and healthy).  Any thoughts why the nitrites are elevated and what is the best way to correct this?  Thank you.

Answer
Hi Jennifer,
The bad thing here is ( I don't mean to be too blunt) but you did a very major overcleaning. Which should not have been done.
Because now you'll have to re-cycle and re-establish your aquarium. I would try to get rid of any coppersafe and any other chemicals by doing daily 50% water changes and use a quality water conditioner like Amquel+. Also use Carbon in your filter to help filter out any additional medications.

The water changes are the best solution to your nitrite problem as well. Your aquarium is going to have to establish the beneficial bacteria all over again. So treat your aquarium as if you had just set it up. Test your water everyday, and make those necessary 30-50% water changes whenever your ammonia or nitrites are out of the safe bounds or whenever your fish seem stressed.

If you have any access to another established and healthy aquarium, taking some of that tank's gravel or filter media and putting it in your own setup can really help a tank that has lost its beneficial bacteria.

Best wishes and I really hope this helps!
Karen~