We bought 4 dwarf red crabs a while back... they lived happily for about 3
days then 2 of them decided to climb up out of the tank and sit on the glass
ledge just under the hood... where they died. The other 2 crabs stayed in
the water and seemed to freeze solid and again die.

Anyone know why this might be?

I would really like to give them another go but obviously i don't want them
dying again. Since then i've cleaned the tank out throughly and replaced the
foam filter with an undergravel one with power head one end and riser the
other with air stone and carbon filter on.

We have fish in the tank that are all healthy, just as we did when we had
the crabs.