Cycling help- nitrites keep increasing

[kittykaboom]

Hi there, looking for input and advice on my cycling situation. I began cycling my new tank (40 gal breeder) in early November in preparation for an axolotl. Things were going as expected.. until they weren't. During weeks 3/4 parameters were encouraging with ammonia returning to 0 within 24 hours, increasing nitrites, and increasing nitrates. I have been using the API Freshwater Master Test kit since the beginning.

The problem I'm having is that I'm stuck in this step now. I can add ammonia to 2ppm and it's back to 0 in less than 24 hours which is great, but the issue is the nitrites. The nitrites are consistently high, definitely at 5+. Currently I am not adding ammonia at all because the nitrites haven't reached 0, not even close. I have gotten the nitrites down to 0.50ppm with large water changes (75% or more) but then the next morning they'll be back up to at least 5ppm.

What could be causing constant nitrite production like this? My tank has about an inch of very fine sand as substrate (thoroughly cleaned prior to being put in tank), driftwood pieces (thoroughly cleaned), a handful or two of aquatic plants (all in good health, not melting), plastic 3D printed hides, and some slate rock.

I'm running an AquaClear 50 filter full time, tank temp ranges between around 64-68 depending on how high I crank up the heat, and I have plant lights above the tank. I've been using Dr. Tim's ammonia, Seachem Prime, and API QuickStart.

Parameters this morning:
pH: 7.4
Ammonia: 0
Nitrite: 5
Nitrate: between 5 and 10

I am just feeling really defeated. We've now had our axolotl since the second week of December, who has been content being tubbed, but Godzilla is looking forward to moving into his big beautiful tank!

 

[wolfen]

if you are only keeping one axolotl then the filter is just large enough (with a little altering of the media).
for the filter.. get a sponge for the inlet pipe, at least half the filter media should be bio-media, remove carbon pad whilst cycling as it's not required.
for the tank... make sure that the water is oxygenated (the bacteria require oxygen as well as your axolotl), for a forty gallon at least two air-stones required, make sure the sand is rotated regularly (debris can build up, as well as bad bacteria), keep the tank warm when cycling as it encourages bacteria growth.
to cycle a tank bacteria that consumes/converts ammonia and nitrites need to grow, you have/had enough bacteria that consume/convert ammonia but you haven't got a big enough amount of bacteria that consume/convert nitrites, the reason I said have/had enough ammonia converting bacteria is because you have stopped feeding them so they will reduce down.
keep topping up the ammonia daily and increase the bio media in the tank..
this video may help

even though it's for a 70 the idea is the same.

 

[kittykaboom]

if you are only keeping one axolotl then the filter is just large enough (with a little altering of the media).
for the filter.. get a sponge for the inlet pipe, at least half the filter media should be bio-media, remove carbon pad whilst cycling as it's not required.
for the tank... make sure that the water is oxygenated (the bacteria require oxygen as well as your axolotl), for a forty gallon at least two air-stones required, make sure the sand is rotated regularly (debris can build up, as well as bad bacteria), keep the tank warm when cycling as it encourages bacteria growth.
to cycle a tank bacteria that consumes/converts ammonia and nitrites need to grow, you have/had enough bacteria that consume/convert ammonia but you haven't got a big enough amount of bacteria that consume/convert nitrites, the reason I said have/had enough ammonia converting bacteria is because you have stopped feeding them so they will reduce down.
keep topping up the ammonia daily and increase the bio media in the tank..
this video may help

even though it's for a 70 the idea is the same.

Thank you so much for the thorough reply and for sharing the video! This is all super helpful. I didn't realize the carbon pad wasn't needed in the filter, so will remove that and will add some additional bio-media. I'll add air stones to the tank as well.