The folks at New Life have based their inclusion rates of garlic on actual science, and tried & true methods. Feeding garlic once a month will have little overall effect on the health & well being of your fish.
http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1678-91992006000200003&lng=en&nrm=iso
While the document linked to above is a very long read, in the end the authors state:
A. M. Shalaby et al. EFFECTS OF GARLIC (Allium sativum) AND CHLORAMPHENICOL ON GROWTH PERFORMANCE, PHYSIOLOGICAL PARAMETERS AND SURVIVAL OF NILE TILAPIA (Oreochromis niloticus).
Fish Hatchery and Reproductive Physiology Department, Central Laboratory for Aquaculture Research, Abbassa, Abo-Hammad, Sharkia, Egypt
J. Venom. Anim. Toxins incl. Trop. Dis., 2006, 12, 2, p.196
In this particular study the inclusion of garlic at a rate of 3% has shown to increase the overall digestibilty of protein, carbohydrates, and fat (ie; better growth), as well as to lower the total bacteria count within the intestine, muscles, and water column.
And another recent study ..........
Effect of Allium sativum on the immunity and survival of Labeo rohita infected with Aeromonas hydrophila
S. Sahu, B. K. Das, B. K. Mishra, J. Pradhan and N. Sarangi
Aquatic Animal Health Division, Central Institute of Freshwater Aquaculture, Kausalyaganga, Bhubaneswar, India
Journal of Applied Ichthyology, Volume 23 Issue 1, Pages 80 - 86
As previously stated, there is no need to add garlic to NLS food.