Nitrite Showing After Filter Change

[Harlequins]

In my cory fry tank,i changed the internal filter because it was too powerful for the wee fry,so i changed it for a smaller hob filter and used all the mature media from the original filter,now i keep getting nitrite spikes...
I would've used my sponge filter but its not cycled anymore...

It started with just a tad showing,this morning it had shot up to 0.25ppm and that was after cleaning the bottom of the tank and doing a small w/c 11pm last night using the water from the adult tank,which the stats is spot on (zero ammonia/nitrite).
So this morning i done a 75% w/c using fresh dechlorinated water this time,carefully matching the temperature,an hour later tested it and again its showing a tad of nitrite??
I thought it might be the testing kit,so i tested all the tanks and its only the fry tank thats showing nitrite
I don't overfeed them and test the water before a feed,i just can't work out why its happening...

Any ideas?

Edited to add the stats were spot on before filter change,and nothing has been added...

 

[waterdrop]

Hi Harlequins,

The little structures that bacteria build up in their biofilms are now known from research to be actual little "structures!" There are sometimes little calcified tunnel-like patterns and the purpose is thought to be fairly clearly a way to increase the exposure of imbedded bacterial cells to more ammonia and fresh oxygenated water. When mature media is physically disturbed, especially if moved and fitted to a new filter (at least one can imagine) there is the chance that if these scientific observations are correct and the functions being correctly described... that some of these little biofilm structures will be crushed and/or entire sections of biofilm loosened and washed about. This is not necessarily a bad thing in the long run but it could cause a noticable gap in full functioning in the short term.

OK, so that's the fancy thought, but the more prosaic explanation may be simply that you've stirred up a significant amount of sedimentary material during the gravel cleaning and other changes. Everything that gets stirred up when you do these sorts of things is not necessarily visible. A lot of ions and organics are charged molecular constructions and likely to find things to loosely cling to when they are down in the area of the gravel or have been trapped in the beginning parts of the filter. When you make big changes these things can be release more into the general water column.

It can take some days for all this to settle down and it may take some days for your bacterial colonies to refresh their biofilms and overcome your little mini-cycle event. You are showing that you're a good aquarist by being on top of your monitoring and knowing to perform the correct type of water changes during an event like this. I hope it will all settle down for you before very long and I think that is the most likely direction things will go.

~~waterdrop~~