You need to put a thermometer in the brooder and verify actual temperatures. One cause of lethargy is too high a heat. I am also concerned about the rabbit hutch covered with plastic and holes for air--it could be getting too hot, or there might not be enough air. I would suggest covering with blankets or towels rather than plastic.
Another possibility is lice/mites. They will kill babies much faster than adults, and the symptoms are similar to what you describe.
Since the birds all come from the same hatchery, that could be the problem.
Personally I do not like brooding that many babies together--20 is the absolute maximum number of babies I will brood together, and I prefer half that number and two brooders.
My recommendations are to
- thoroughly disinfect the brooders with oxine, vircon or novalsan. When dry,
- thoroughly spray it with a premise spray such as Adams. Get the kind that is effective for 7 months.
- let set for 24 hours after dry
- disinfect all feeders and waterers. I clean then soak in a strong bleach solution for an hour or two.
- feed medicated feed and
- use ACV in the water, but
- do not use corrid or sulmet unless you see symptoms of coccidiosis.
- Get an indoor/outdoor thermometer with a wireless remote.
- Put the remote in the lighted brooder and
- monitor the minimum and maximum temperatures daily for several days before adding chicks, and each morning after you add them
- if you work or are gone from home regularly, check the min and max temps when you return. If you are nearby most of the time, check the current temp several times during the day.
At the first signs of listlessness, check in the wingpits for mites/lice--use a strong light and a magnifying glass if it will help you see better. If they have them, immediately spray treat with flea/tick spray such as Adams.