I'm glad your brother's little one is ok now, i was worried there for a while but all is well
As everyone else said, bath them for longer. When I bath ours I don't leave them in for as long as 20 minutes, but I leave them in for longer than 3-4 mins. It does them no harm to have a long bath, and they can really learn to enjoy it. Our adults love to swim around

Our adult male used to poo every time we put him in and really hated the water but he has come round to the girls' way of thinking now and just today was kicking back his legs and swishing his tail, nudging the girls out of the way as he swam like a fish
Our new younger girls hate the bath, but even when they try to get out, leave them in, they will calm down. Don't leave them unattended though. Dragons will often puff out their bodies in the water so that they will float, but there is always the danger of drowning, so don't wander off while it's bathtime.
Looking forward to seeing the new tank!

How large is it in feet and inches? (Sorry, no good with gallons!!) I'm sure the Bearded Dragons will love it!
Great job in helping out your brother's dragon,. well done.