The Star's Peter Calamai reports in his "This week in Science" column that the Canadian and U.S. governments are trying to staunch the flood of aquatic invasive species into the St. Lawrence Seaway and Great Lakes by cracking down on ships that pump out ballast water loaded with foreign stowaways. But there's another danger closer to home: private aquarium owners who flush fish down the toilet. Using old-fashioned legwork, three McGill University environmental researchers, Erin Gertzen, Oriana Familiar and Brian Leung estimated that 10,000 goldfish, guppies, Siamese fighting fish, and two score other aquarium-dwellers are set free by their owners in Montreal over a year. At least 18 species are loosed in high enough numbers (100+) to propagate a resident population in the Seaway - except that winter water temperatures there are too nippy for most aquarium fish. Yet cold-tolerant goldfish and Koi carp are already happily established.