The reduction in your frogs alone is not responsible for the increase in the slug population. Everyone has had far more slugs this year, simply because of the wet weather, which has encouraged them.
If you've got a frog which is happy with your water feature, then other frogs would be too. But it's always useful to have plenty of water sources. It's not absolutely necessary to have a plant in it, though any frogs and tadpoles would certainly appreciate it. If you want something, try a bunch of Elodea (available from garden centres, and from Amazon, 10 bunches for £4).