Sounds like chicks are being overheated. And overcrowded. By 2 - 3 weeks of age, chicks should have a minimum of 2 s.f./bird. And by the time they are 3 weeks old, if in the house, they should be completely weaned off heat. The brooder should have a warm foot print of space, and the rest of the brooder should be at ambient outdoor or room temp. If brooding in the house, they should NOT have a heat lamp (standard 250W). That is way too much heat. Combine that with a plastic tote, and you have an easy bake oven. There is no way for the chicks to move away from the heat enough to cool down, and it slowly kills them.
Electrolytes should be a one time use. Using them all the time messes up their electrolytes. It would be like you drinking NOTHING but gatorade day after day.