Tubs on a temporary basis are okay but lack of ventilation and reaching down in over their heads makes them stressed. And obviously the concern with heat. The fact that you turned down the heat should really help. It's probably a combination of shipping stress and then heat. Just keep a close eye on them. Behavior shows you what temps to use. If they're not acting cold, go lower and lower. What I use for a brooder is hardware cloth screens from a pre-fab coop and OSB as coop floor. The older and messier they get, the tub just holds in moisture too. So OSB with paper towels and then pine shavings is what I use in my brooder. I use heat plate and ceramic lamp for first days, then a MHP and ceramic lamp out in my coop brooder. I just took the ceramic lamp away for my 6week olds. It's *30-40 night temps.