My advisor's area of expertise is immunology. I work in his lab and every project involves some aspect of some kind of fish disease. Believe me, if garlic had any positive scientifically supported effect on the fish immune system, he would know all about it. But in fact he is quite outspoken against it specifically because there is not scientifically supported evidence, not even a hint of a biological mechanism, that garlic does anything to boost the immune system. He's done plenty of research on it himself- all the evidence suggests it just doesn't do anything helpful.
On the other hand, there is both evidence AND a known mechanism by which garlic can cause harm to fish. Terrestrial lipids in general can be a problem for fish because they do not have the enzymes to break them down. And why should they? It's not something they usually encounter. Instead the lipids get deposited elsewhere in the body which can lead to serious health problems down the road. It's much the same reason that High Fructose Corn Syrup is so bad for us- we don't have the enzymes to properly digest it so it gets stored, or broken down improperly, leading to long term problems.
I get that personal experience is important- it itself is a part of the scientific process. Observations of things like the seeming immune benefits of garlic are what lead to real knowledge through controlled experimentation. But we can't cling to observations and experiences if they contradict experimental evidence.
And yes, salmon were used as subjects for most of those, because unfortunately the amount of funding for tropical fish research is paltry in comparison to the unholy sums pumped into food fish aquaculture research. I know you might be thinking that maybe it only has that effect (or lack of effect) on salmon, but until we test other fish we can't know for sure. What we do know is that at the very least, clearly garlic is not effective as a blanket treatment or immune booster. We also know that it likely causes at least some long-term harm. So unless research is done showing a clear benefit to other kinds of fish that outweighs the long term consequences, I will continue to advocate that garlic is unhealthy for fish.