I have a pond with no filter but lots of plants. You can see it on this thread. Have you got fish or newts or any other predator in the pond? The spawn does deteriorate and the tadpoles just look like a mass of black blobs. I have tadpoles in my barrel and the black bird was picking them off so we covered it with net. My water went cloudy too. I put some tadpoles in an old fish tank in the greenhouse they are growing well. There was also some in the pond but there are fewer every time I look. I saw a large dragonfly nymph today and what with the newts mine don't stand a chance in the pond. Maybe next year you could save some until they get a bit bigger.
I found Toad spawn yesterday. Nothing seems to eat that apart from the water boatmen .
Wildlife Gardening Works !!!