According to whom? Certainly not those in the scientific community that have been researching garlic and its various compounds, and running feed trials on various species of fish.
http://www.monsterfishkeepers.com/forums/showthread.php?458994-Garlic-and-the-Prevention-of-Disease
That should help clear a few things up on the subject of garlic, and aquaculture, and what we currently know.
And here's another recent paper that you can add to the list posted in the link above.
http://idosi.org/wjfms/wjfms1(1)09/1.pdf
HTH
