Rather than look for poisonous algae, why not look at your water parameters and the requirements of the fish you have been buying? Although BGA is indeed poisonous to many fish, as Bignose said few fish will try to eat it. That leaves us looking for another reason that you are losing fish. Some of the other factors that are not immediately obvious are the mineral content of your water and the corresponding pH. Mineral content of water can be a bit tricky to work with since the readily available information about the fish seldom includes that data. A guide that I end to use is the hardness and pH numbers that are often well known. Water with a low pH that is fairly soft I read as water low in TDS in most cases. Water that is hard with a high pH I read as high in mineral content. Once I have decided that a fish requires a high low o]r medium mineral content IO set about trying to achieve that condition. If my own tap water has a high mineral content, I decide how much rain water I will need to mix with it to get a low enough mineral content for the fish. If my water is fairly soft, not the case I am dealing with right now, I look to bring the mineral content higher using things like crushed shell or crushed coral in the filter to introduce some calcium carbonate into the water. Water must be matched within reasonable limits to the fish or you will have problems keeping the fish thriving.
