Reviews on that product are mixed. The majority use it without issue and give good reviews whereas a small proportion of users report issues similar to what you have experienced. There will be some other factor involved as to why the fish died for you whereas other people have no problems with it. Maybe its reacting with something else in the tank, maybe it is having this effect on particular species and not others, maybe there is another cause completely not related to the product you used and its coincidence.
Knowing some more about the tank, water parameters (before all the water changes), how long its been set up, is it cycled, any other new products you used, what precisely you did with the product etc will help rule out any other cause.
Its just an algaecide, water changes should remove it just fine. Only real way to know is to get some more fish. If you want to be ultra cautious, break everything down, bleach treat what you can, replace what you cant.