Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
What I would do: (Exactly what I did a month or so ago!)
100% + water change. Dig into the substrate a bit to drain out that last little bit.
Then a little bit of refill and drain it again. (Rinsing the tank decor of any lingering nitrogen.)
Clean the filters. (gently rinse and reuse the media)
Then refill the tank with just ONE dose of dechlor. Do not overdose.
Check the KH, and maintain at least 3 German degrees of hardness.
Then add BARELY enough ammonia to bring it to 3 ppm, no higher, and 2 ppm is better than going too high.
Let that run for 24 hours and test the nitrite and ammonia, and KH.
If the nitrite is over 5 ppm then skip dosing the ammonia. You will not starve the bacteria by making them wait a day to be fed.
If the ammonia is low, and the nitrites are low, then dose the ammonia to BARELY reach 3 ppm.
If the KH is dropping add baking soda.
Next day, test again. Ammonia and nitrite. Continue dosing just a little ammonia, never too much, until the nitrite removing bacteria catch up. Might take a week.
Then add a little more ammonia to really reach 3 ppm.
When you can add enough ammonia to reach 3 ppm and the bacteria remove it all overnight (24 hours), and the nitrites are zero, the cycle is complete.