You need to find out what the temp is inside their brooder it might be best to turn the light off if it's too hot. Actually, I start my ducklings out at 85 and drop it 5* each week there after.
I think they are too big for the tub. Anyway you have an appliance store close by they might have a huge box you could use?they are so adorable😍. It does take up a lot of time when ducklings, cleaning, feeding.