Seems to me it must have been a salt water tank if it had clownfish, and they are tropical so it's not just exposing the acrylic to constant warm water, but a more realistic marine environment.
In the lab I've chemically tested acrylics that were bullet-resistant at low-cm window thickness. No bullets were used in my phase of the operation, but I was clipping off chips from the exact coupons which had been physically measured beyond the fracture point.
You can tell there's a difference in ductility and brittleness with temperature. Pretty tough stuff regardless.
In the aquarium these are supposed to be very thick-walled acrylic curved panels bonded together.
If the water was maintained warm enough for the reef fish while the ambient air dropped below freezing I don't like that idea. The entire temperature differential being borne by the thickness of the acrylic could mean there are different plastic properties diverging among the inner and outer surfaces of the "glass" panel. This could give rise to stress being concentrated deep within the panel if the transition is not too smooth.
Conditions like this could be recreated if so.
Might have been OK if they had penguins and cold-water fish this time of year.
