Fish will eat tadpoles if they can get to them. The only way to stop them would be to give the tadpoles a space that the fish can't get to, like a shallow area with a barrier (plastic mesh would work but make sure the fish can't get caught in it) or lots of plants, or make a separate wildlife pond.
Most garden centres sell plastic mesh of varying sizes. The flat, smooth kind would be better rather than the thin, thready kind because it'll be less likely to injure the fish. If you can get fabric netting that would work too although it might be trickier to get it to stay put.
The other thing you could do, if it would work in your pond, is to weigh down a plastic crate with a couple of bricks and move all the tadpoles into that. You'd need to find one with the right size holes and the right depth though.
Whichever one you do, I would chuck a few handfuls of elodea or hornwort in as well to give the tadpoles somewhere to hide and somewhere to sit when they become frogs. You might have to get plants from the indoor section of the aquatic shop, it's a bit early in the year for pond plants. Tell them it's for a pond so you don't get something that'll die off in the cold.
Wow I've made a tadpole nursery. I used a plastic storage tub with holes and hot glue gunned net all around the outside. I just put it in the pond to secure it on a shelf in the pond but to my surprise it floats just level with the surface of the pond!