seems a bit early for frog spawn, I have had a pond for 4 years, had frog spawn from year 1, not got any in my pond this year yet, but you want to watch out for dragonflies, my tadpoles over the year kept reducing until I saw the dragonfly larve eating them, also cover your pond when you have tadpoles, blackbirds eat them like sweets. I have also got newts in the pond