How big were they when you started to fertilize? I usually wait at least until the first set of true leaves develop to start fertilizing. They might come back, allow them to dry out between waterings from now on. Also, they like temps on the cool side, are they in a warm area?

Fertilizing twice a week is too much. Water only when they are dry and not just dry on top.If they are a dull yellowy green you are probably giving them too much water. One indicator is weight. A dry plant container will be light, a wet one heavy. You can stick your finger in the soil and if it feels wet half way down, it doesn't need water. More seedlings die from too much water than those who get too little.

Do not fertilize until they have their second set of true leaves and then only every 2 weeks A mature plant can wait 4 weeks between fertilizing.

They are in the basement and started out great but I got overzealous with the watering can. Should the growlight be right on top of them, the same as tomatoes and peppers? I was watering from the bottom when the tops of the container looked dry. It wasnt down in the soil mix below. I have drastically cut back on water now. They did have second leaves and are now about 4inchs tall in 4inch pots. Hoping they will recover but started some backup anyway, dont know what ill do with 100 brocolli plants if they all survive, lol.....

Wow, yea you will have your hands full with all the broccoli! I would keep the light close to them the same as the others.