Not sure if this is mentioned in the abstract, but for the fish that were fed the effective doses of garlic (10% and 20%) a very large portion of the fish sampled after that procedure showed muscular dystrophy after just 14 days of this experiment.
"Evidence of muscular dystrophy was observed in all sampled fish. Ten out of 11 examined fish from the group fed 20% garlic displayed evident dystrophy, which was slight in 8 fish (about 3% of the muscle tissue seen in a section was affected), and more severe in two cases (about 10% of the muscle tissue). In the group fed 10% garlic 7 out of 11 fish displayed muscle dystrophy and in the continuous bath treatment and control there was the least occurrence of dystrophy, (2 out of 11 fish and 3 out of 9 fish, respectively; data not shown)." (Fridman, 2014)
AH, Sargent JR, Thompson KD 1993
Terrestrial and fish oils affect phospholipid fatty acid composition, development of cardiac lesions, phospholipase activity and eicosanoid production in Atlantic salmon (Salmo salar).
Prostaglandins Leukot Essent Fatty Acids. 49(3):665-73
AH, Park MT, Sargent JR.1991.
High dietary linoleic acid affects the fatty acid compositions of individual phospholipids from tissues of Atlantic salmon (Salmo salar): association with stress susceptibility and cardiac lesion.
J Nutr. 121(8):1163-72
J. Gordon Bell2, John McEvoy3, Douglas R. Tocher, Fiona McGhee, Patrick J. Campbell* and John R. Sargent 2004
Replacement of Fish Oil with Rapeseed Oil in Diets of Atlantic Salmon (Salmo salar) Affects Tissue Lipid Compositions and Hepatocyte Fatty Acid Metabolism
The American Society for Nutritional Sciences
J.G. Bell1, D.R. Tocher1, B.M. Farndale1, A.H. McVicar2 and J.R. Sargent1 1999
Effects of essential fatty acid-deficient diets on growth, mortality, tissue histopathology and fatty acid compositions in juvenile turbot (Scophthalmus maximus)
Journal Fish Physiology and Biochemistry 1573-5168 Volume 20, Number 3 p263-277
SL Seierstad, TT Poppe, EO Koppang, A Svindland, G 2005
Influence of dietary lipid composition on cardiac pathology in farmed Atlantic salmon, Salmo salar L
Journal of Fish Diseases.
Those are my go-to references regarding garlic. (Most) Fish don't have the enzymes to break down the lipids found in terrestrial plants, so they end up getting stored in the body. So it's likely that even a tiny amount can be harmful over time as it will continue to be deposited rather than digested or allowed to pass through.