A few questions.
Do you have a big enough heater to hold the temperature in that small tank stable in a cold house?
Is there a filter for the small fry tank?
Is the filter already cycled or did you set the tank up in a hurry for the fry?
Some of my thoughts are that the cold air may be affecting the tank temperature. Air flow will also stir the water and any uneaten food will tend to decay faster. That would not be a big problem with a cycled filter but, if you have a new tank, it could mean a rapid build of ammonia in the water. If your tank is newly set up, the only way to avoid ammonia harming the fish is to do frequent large water changes.