Algae is typically a summer time problem. 

So are high temperatures. 

This two things in combination make treating an algae problem during very hot periods can be a tricky thing.

The problem is that as the water temperature rises, it's ability to retain dissolved oxygen begins to go down...as does its maximum saturation point of DO.  

When an algaecide is applied in order to kill algae (the same could be said for herbicides on aquatic weeds) the plants will pull oxygen from the water as they die off.

And this can create very rapid oxygen drops and even depletion in a pond environment.  It should go without saying, but this is not good for fish.

Mass fish losses can and do occur because of a poorly timed application of a killing agent like an algaecide.  

The key to avoid having a catastrophic impact on fish is to make sure you have robust aeration going during and after the treatment.  The aeration system should be run 24/7 with no down time.  

If the algae has some mass to it like string algae, you can try to treat it in sections or smaller areas, and over a longer period of time, rather than one broad, all-encompassing application.  In this way, the die off process is staggered and the effect on oxygen is more limited.  

Finally, it may be wise in some instances to just wait and pick a cooler day to treat the algae.  

This is just a friendly reminder to take care when using any kind of algae during periods of hot weather.