I used RCS which I have plenty of culls for, since they are more sensitive to most chemicals than fish.
I have elephant noses, which I can assure anyone, are some of the highest respiration rate and sensitive to chemical fish livestock out there. They where fine even if you treated 3 days in a row.
Algaefix is not suitable for GW from what I've seen and heard.
Hair algae and that's about all.
Plant cells and animal cells are extremely different on several major factors.
This is a Biology question on many exams: "What ways are plant and animal cells different? List 5 key differences"
Equating them is a huge stretch without support.
The EPA's Environment toxicity report on aquatic organism is a bit like the NO3 issue, it's for Blue Gill and trout, trout are extremely sensitive vs a war water tropical fish species. Even so, the EPA report suggest that fish are more at risk to dose than invertebrates(they tested Daphia only).
Plants are not at risk, going about 73ppm, vs 0.28 for Daphia and 0.21 for Blue Gill.
Duckweed is .63 ppm or so.
Without specific test on specific species, it's hard to say. Shrimp are definitely far more sensitive to this product than ANY fish. I've tested this and see no evidence of any fish issues with perhaps 20-30 species, many of which are touchy species compared to things like guppies. Tetras, rare plecos, elephant nose, small cichlids, typical stuff plant folks keep. It is more toxic to green algae that diatoms of BGA, 6x to 100x etc more so.
Most of the diatoms are about like the sensitive fish in terms of toxicity.
CO2 kills fish to, but plenty of people use it
If you do not use it correctly, you will kill your fish with CO2, plenty/many do.
Point is: shrimp will die long long before any fish does. I also tried the 1/2 dose, this had no difference on the algae(still killed it effectively), and still killed a few RCS. Fish had no issues.
Fish stress when you dose CO2 also, but we still have people dosing CO2 24/7 without the same critique
Similarly, we add Excel and CO2 to shrimp tanks, but that reduces the fry production.
How much risk you want to apply/accept is something we all do in this hobby. Suggesting that it's too risky is no different than people saying CO2 is too risky. It is very specific to hair algae and thus a 1-2x treatment type of thing for a few days. So it's not something hobbyists use 24/7/365.
Note: Algaefix and Algae Destroyer(copper based) are 2 very different products, I've sure a few folks have thought they are the same thing.
Toxicology test suggest 2-3x a Excel dosing as Glut is very toxic to many species also, but many do it.
Like Glutaraldehyde, Busan 77 is also a microbiocide. Same with H2O2....and you can kill fish and stress them using those products as well.