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The fact that they are dying overnight points to oxygen deprivation as a result of the green water (algae) consuming oxygen at night. You mention high temperatures, but do not expressly say what the water temperature is. If the water is 90F, you have a real problem. The worst is still to come! Water temps over 80F, plus heavy green water necessitate aeration. Fluctuating pH due to low KH may be playing a role in stressing the fish.
Dealing with the green water is a challenge. First, if you use one the algicide products there will be a great drop in oxygen content that will kill fish. Second, if you use UV to control the algae, be prepared for ammonia/nitrite problems. It takes 6-8 weeks for the biological filter to cycle. The greenwater algae are consuming the ammonia. As a result, when the algae are eliminated by UV, there might not be a sufficient population of nitrifiers for the number of fish for a few weeks.
Question: How frequently does your filter turnover the volume of the pond? I recommend once per hour. The conventional wisdom is once per two hours.
The greenwater will clear "on its own" (or at least become much more mild) when the filamentous algae get established on the pond walls and the filters truly mature. Meanwhile, add oyster shell (crushed oyster shell is sold as chicken grit) to raise KH and add extra aeration.
Good luck!!
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