We've a 3000 gallon koi pool and a small frog/lily pond.

But we've had frogs in our garden from before we had either.

In my younger and less affluent days I used to service my own car.
I kept a 1ltr ice cream tub, which I used to catch the engine oil when I drained it. After disposing of the oil, I put the mucky container with still some oil in the bottom behind the garage. Once, a few months later I noticed two frogs in about two inches of rain water in the tub and "at it."
They have more comfortable accommodation now, the three foot pond they use can contain up to a dozen pairs each year. The spawn sometimes covers the whole of the surface, but each batch looks slightly different to the next.
I've read somewhere that on average, only two frogs reach maturity from any spawning.

If you've a pond, make sure the baby frogs have some way of getting out when it's time for them to leave by placing some rocks they can use to assist their exit.
Tadpoles like fish food. I feed them koi pellets which they nibble at and chase round the pond.

I've already cleaned out our little pond as it'll be soon be time for them to pair up.
We never see any slugs in our garden.