You say they get weak. What else? Do they pant? Stand with their wings out? Huddle together? Stand crouched with their eyes closed? Shiver? Run around? Peep?
Do you have a thermometer in the bottom of the brooder so you know what the temperature is where the chicks are? It needs to be about 95°F the first week. At least part of your tank needs to be that warm, part of it can be a little cooler so they can choose how warm they want to be.
Do you change their water at least once a day and give them fresh food daily? Don't give them cold water at first. And when you first get them be sure they drink before feeding them so they're well hydrated first. Are you feeding them anything else besides the chick starter? Don't, at least for the about the first week. Provide chick grit in a separate dish before giving them anything besides chick feed.
Keep their bottoms clean. If they get plugged up with pasty butt they can die. Wash their bottoms with warm water as soon as you see any sign of it. Check them daily.
Keep them warm at night as well as during the day. If you use a red bulb, be sure it is rated safe for livestock and reptiles. As another poster said, some red bulbs have a Teflon coating and is deadly to birds because it produces gasses.
Don't handle them too much. Baby birds need lots of rest so they can grow, and they are fragile.