This is from bob Groemans site
Early experiments with the product showed that while much safer and less toxic than other traditional treatments of the past, it did have a negative effect on alga. Therefore, aquarium alga and invertebrates containing zooxanthellae were at risk, which were witnessed firsthand in some early experiments, where some soft and stony corals perished (L. Ichinotsubo, pers. com.). Nevertheless, Richard Terrell of the Pittsburg Zoo Aquarium & PPG Aquarium has reported using chloroquine-dosed gel foods to halt an outbreak of Cryptocaryon in a giant clam exhibit, with excellent results and credits Robyn Doege of the Dallas World Aquarium for providing the information.