Sorry to hear that. :( Sounds like you just have too many fish for a pond that size. I only have about 25 fish in mine (approx 5000 gal). Did you test the water for nitrates, PH levels and all that stuff?

lack of oxygen? a lot of algae + a lot of animal life will use up all the oxygen. Do you have an air pump or anything in there to oxygenate?

Shade will help to control algae. Is your pond in full sun?

Way too many fish for such a small pond, they died from lack of oxygen in the water. Yes, the tadpoles added to the overstocking but even half those fish are too many.

You may have to many fish, and will need to deal with that. However the timing of the problems and adding the polyester wrap seem to close. I would make sure the polyester wrap does not have any contamination on it, where did you buy it? was it wrapped? Maybe remove it & see how things go.
It also could have restricted water flow limiting filtration, exacerbating your over population problem.

Yes, was your pump still working? Quilt batting works very well but clogs up very fast. I'm wondering if there's a reduced water flow going on

I had the same problem. My pond is a very small starter pond, so 12 fish were plenty to start with. Every one of them died, 3 x. After returning to Pet Smart to refresh our supply, we had taken a sample of water for them to test. The water was fine, from the initial chemical remover from the tap water. We were trying too hard; the plants were what were needed. They need enough plants to be able to hide in and under, no sunlight to peek through. With the purchase of several plants, luckily we have live fish.