Bristlenoses are not cold water fish. Otherwise they'd be banned by MAF because they wouldn't want them to be released in the wild, breed and alter our eco-systems.
Haven't you ever wondered why they don't naturally occur in NZ waters? Um because maybe, just maybe it is actually too cold for them?
Now there are some exceptions to the rule... ie A-town and a few others that keep tropicals outside etc.
But generally they are tropical fish that require 22 - 26 degrees. And is the exception and not the rule.
Kept outside this range is down right cruelty...
Common sense is required when other critters lives are in your hands, they can't move themselves to water that is warm enough for them, they rely on you.
Just remember, if they don't live in water that cold naturally in the wild... there is a reason.