Why won't my malawi fry tank stabilise?

I have had a small 22ltr fry tank setup for sometime now i started by running my Rena iv1 in my main tank for 3 weeks and then when my maingano had a mouthful of eggs i transfered the filter into the fry tank put in a couple of convicts untill i was sure the levels were OK and once i had 0ppm across the board i replaced the convicts with the maingano and kept a check on the levels which where fine untill i started to place small amounts of food in the tank to encourage her to spit out the young which she did and she also started to dig but as soon as she felt i was getting to close she would swim about frantically picking up her young. Untill one morning i checked on her and i couldn't see any young i thought the worst so i checked the levels and my nitrite had risen to 0.50ppm after catching a glimpse of one of the young under a rock i removed the mother put her back in the main tank and found all 10 of the maingano young not doing to well under the rocks what had happened was she was burying the uneaten food which caused the nitrite spike so i hurded the babies into a cup and placed them in a breeding trap in the main tank untill i got my levels down to 0ppm i currently have the babies in the fry tank but i can't get the ntrite to stabilise its either 0.25ppm or not quite 0ppm i am removing any uneaten food before next feeding time but still no joy, Please Help.

Cheers, Derek.

Cheers for the reply but their are fish in the tank the 10 maingano young are in their and when i removed the mother and young the first time i transferred the filter back into the main tank and washed the gravel in a bucket so to make sure i had removed all traces of uneaten food i realise that i would have upset the bacteria in the gravel but it has been running long enough now to have built up more bacteria. just checked my nitrite again and still not quite at 0ppm i am using the API freshwater test kit which i am led to believe are very good. Cheers, Derek.