This is why it is vitally important that 80%-100% water changes be done DAILY during tank cycling.
(What is tank cycling? The most important aspect of keeping fish! Have a look here)
These readings are before water change -> after water change. (and the reading is .25 after, not 0)
As you guys know, I just moved. And in moving my tanks, their cycles had to re-establish. Two of my three tanks are already back on track, but this tank had JUST re-established before I moved so it was more fragile and the nitrite eating bacteria died off.
I have been doing 75% water changes every 12 hours and STILL the nitrite climbs to serious levels in that time. (I also dose prime in the tank so don’t worry about the babs).
That high reading? Is again, just 12 hours after the last change. I’m livid because I seem to have completely lost my nitrite-eating bacteria.
(I’ve now put some seeding material from one of the established tanks in, so it should get better soon).
But the point is, if you are forced into doing fish-in cycling by circumstance (bought the fish in ignorance, or rescued it) daily water changes are 100% necessary. Don’t miss even one.
And this is also why you should never just USE live fish to cycle a tank - imagine NEVER doing water changes, through this? Torturous.