Dealing with nitrite in an established tank

[Clack]

My 29g tank's been set up for around 6-7 months now. It's home to 9 cories, 12ish pygmy cories, and 4 ADF's. The tank's not super planted, but it's got a decent amount of greenery. I added some extra java fern on 17/12, but that's about it for recent changes. The substrate's sand, and I do a 60-70% water change every 8-9 days. Usually, my ammonia, nitrite, and nitrate levels are all 0. Oh, and I've got an air stone running 24/7.

I did a routine test 3 days ago and was surprised to see nitrite levels at 0.5ppm and nitrate at 40ppm. I did a 50% water change that day, removed some decaying plant leaves, and checked for any dead fish (found nothing). I also vacuumed the sand thoroughly. The next day, I tested again, and the levels were still the same, so I repeated the process. I tested again yesterday, and the levels were still the same, so I did another water change.

I tested my tap water yesterday (after letting it gas off for a few hours), and the results were: ammonia 0, nitrite 0, nitrate 10ppm.

The only thing I can think of that might've caused this is when I cut my filter sponge in half a few weeks ago. I put one half in my other tank to help with cycling and added some new sponge to this filter. I did give the filter a good squeeze during my last water change on 17/12, so maybe I inadvertently caused a mini crash? None of my fish seem sick, though - they're all eating and swimming normally. I'm hoping it's just a matter of doing daily water changes until things stabilize again. I'm pretty annoyed with myself if I did cause this, since this tank's always been super stable.

[coltin]

I'd just keep doing those large water changes until your levels return to zero. It's possible that the filter bacteria isn't evenly distributed across the media, so when you cut it in half, you might've inadvertently removed a large portion of the beneficial bacteria, causing the tank to cycle again.

[Clack]

I'd just keep doing those large water changes until your levels return to zero. It's possible that the filter bacteria isn't evenly distributed across the media, so when you cut it in half, you might've inadvertently removed a large portion of the beneficial bacteria, causing the tank to cycle again.

Thanks for the advice, I'm getting a bit frustrated with all the water changes but I'd rather be safe than sorry. Hopefully it's just a matter of time before the tank stabilizes again.