Finished Cycle Then Ammonia Spike?

[fisshboy]

hi all, quick question.
 
my tank has been fully cycled and has been reading 0ppm ammonia, 0ppm nitrite and 10ppm nitrate for the past few weeks then all of a sudden an ammonia reading of 0.50ppm does anyone know what could be causing this ?
 
many thanks
fisshboy

 

[techen]

How many fish did you add when the cycle finished?

 

[fisshboy]

I ended up doing a fish in cycle, the only fish i have added recently is a black ghost knife a few days ago, i was under the impression ammonia couldnt be present without nitrite once the cycle had finish because the ammonia was converted to nitrite so why do i have this?

 

[eaglesaquarium]

Overfeeding is a possibility, over stocking is another.
 
If that tank is your 200L mentioned in your sig... 

   That's vastly overstocked.  That's more than 60 fish in that tank, some of them are too large for that tank.  (or would grow to large)