My sister used to work for a pet insurence company and they did have people insuring koi carp, but then we are talking about individual fish valued in the multiple thousands of pounds/dollars. I guess if you did have tropicals worth the same ammounts they would probably insure them but only a handfull of species would come into this category, such as extreemly rare arowana morphs like the platinum Asian arowana or South American "snow" arowana, even fish that we class as rare and/or expensive such as zebra plecs and tigrinus shovelnoses are relatively cheap fish when compared to certain strains of koi.