What was the KH? What level do you consider good? What was the pH?
PH swings can kill fish and the level of KH in the water will determine whether or not the pH is stable.
Does the pond have a source of air? As has already been stated, adding air in hot weather can be critical for the fish.
This sounds like a water problem. Clear water has nothing to do with quality water.
We are only trying to help and people come here every day saying their water is fine. But when they post the actual test results of their ponds, it can be far from fine, very toxic actually. So we always ask for actual test results. Sorry if that offends you. But it's the only way WE have to evaluate the situation.
If you are out of strips, it would seems to me that the last strip could have been old. As soon as you open a new pack of those strips and expose them to air, they start to deteriorate, loosing accuracy. So it's hard to depend on the results you get from those.
If you want a more reliable and accurate way of knowing your pond's water quality, please get the liquid type tests. Strips are not accurate and many don't even have a test for ammonia, one of the most toxic aspects of pond water.
Have you added anything to your pond recently, fish, plants, treatments of any kind?
Are your fish.flashing? Do they have any red sores or red patches on them?
Did you examine the gills of the dead fish? If you did, were they red or pale? Were there any sections missing?
Does your tap water have chloramine or chlorine. If it does, what product or process do you use to eliminate that?
We can't help without more information from you.
What questions do you have and want answered?