i think its a spike i have been water changeing one day it be four days then three i was doing the test right this time i am going to drain the tank right down now. the tank was ok has been cyceled for a time with fish in as well
You do realize that your nitrite could be high from the tap right? If that's the case just do one water change and don't feed your fish for a day which should aid you bacs in clearing up the 0.5ppm of nitrite.
I'm not exactly sure what you mean by "turning yellow." I'm presuming it's a api test kit but I have no experience with those. Anyways, test your tap water for ammonia, nitrite, nitrate, and post the results. I'm hoping for you that your water source is the problem here.
If not then something has crashed your bac colony because a cycled filter doesn't allow the nitrites above 0ppm. My guess as to what caused would be one of the following.
. The filter was cleaned recently
. New stock was added and you cycled to less than 5ppm