That sounds like a lack of light intensity. Seedlings need lots of light to grow stocky and strong. Also in warm temps with lots of nitrogen fertilizer they may tend to grow luxuriantly and not form good stocky seedlings. Either move them to a very bright window preferably with some direct sun or suspend two shop lights with one cool white and one warm white florescent bulb each about 4 inches above the seedlings. Run the light 14 hours a day.
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