There are three issues I could see a problem with when using a product such as Alagefix.
First when a large amount of algae dies off all at once, it could possibly create an ammonia spike. Ammonia is more toxic the higher the pH, so in an area like NY where the pH may be higher, these ammonia spikes might be more lethal in comparison to users who have softer water. Most of the so called deaths I see from algacides are usually a secondary effect of the medication. It did what it was prescribed to do kill algae, but it doesn't prevent Ammonia spikes which is what in my opinion could potentially kill the fish. Many of the symptoms you described from the effects of the Algaefix, are also symptoms of ammonia poisoning such heavy/rapid breathing or looking drugged would probably be hyperplasia of the gills. A fish with hyperplasia could simply not process oxygen which is what killed your fish, not Algaefix but a result of algae dying. This is probably the biggest reason in my opinion why some have success and some don't with this product. I'd be willing to bet that if you used the product in the same fashion in a tank without any algae the outcome would be different.
Second algae provides alot of oxygen, which nitrifying bacteria needs to grow. Algae and plant life also consumes Ammonia, Nitrite, and Nitrate in all 3 stages of the nitrogen cycle. So if you take away supplemental nitrification, while depleting the oxygen source for your nitrifying bacteria, at the same time as an ammonia spike is created, you will have problems especially at a higher PH level. Algae also consumes Co2.. which if all the algae is removed all at once, you will have a sudden increase in Co2 levels. Depending on your alkalinity, this sudden shift in Co2 could cause your pH to crash. This is the second biggest variation from user to user that's going to harm fish.
The third biggest difference from one hobbyist to another is going to be water quality, fish that have been exposed to high nitrates over a extended period of time will suffer from numerous health issues. I hate to use stereotypes but I will here anyway, I'll go out on a limb and state that most people that are using this product are more than likely to have less than ideal water parameters(I'm not saying everyone). Higher nitrate levels decrease o2 levels which ties into the depleted oxygen levels during a rapid algae die off.
The product has clear warnings that its harmful to pets and humans so I would certainly transfer that warning to fish and inverts. There are reasons why member such as myself don't recommend chemicals, fish, or other methods to remove algae and perform general tank maintenance.
