If you have a significant algae bloom, the pH will be low at dawn and high at dusk, since algae drops pH at night and increases it during daylight hours. It is not uncommon during algae blooms to have a pH of 5 at dawn and a pH of 11 at dusk. If this is the case, getting control of the algae bloom is critical to stabilizing pH.
However, if the algae bloom can not be quickly brought under control, the "best" secondary approach is to increase both KH and GH in an attempt to stabilize the pH daily swings from the algae bloom. I recommend keeping KH in the 100 to 500 ppm range by baking soda addition and GH in the 150 to 250 ppm range with calcium chloride flake addition to stabilize pH somewhat during algae blooms. The pH swings will still be measureable but the size of the swing dawn to dusk will be dramatically reduced. With high KH the pH at dawn should not drop below 7.5, with high GH the pH at dusk should not exceed 9.5, since at values above 9.5 if the GH is above 150 ppm calcium carbonate will precipitate to drop the pH and avoid extremely high pH values.
With increase in KH alone, the extremely low pH values at dawn will be avoided, but the extremely high pH at dusk will continue. With increase in GH alone, the extremely high pH values at dusk will be limited, but the liklihood of very low pH at dawn will continue.
Low pH at dawn can kill fish at values below 6, small koi are much more sensitive than larger koi to acidic pH values, and die sooner. High pH at dusk is a special problem if the ammonia levels are high, since high pH makes the ammonia more toxic. At very low ammonia levels, the high pH won't kill the fish. If you see the fish piping at the surface late afternoon and evening during algae blooms, the reason is typically high pH combined with significant ammonia levels compromising the gill function to make oxygen takeup more difficult for the fish.
Also, with a high algae bloom, dissolved oxygen at night can become problematical unless there is sufficient aeration to maintain dissolved oxygen content while the algae robs the water of dissolved oxygen all night.