They seem pretty inept. They make their way down to the base of the weed where it's damp, then just hole up there without bothering to go any further. If I'm removing pond weed, I put it into a wire basket placed over the corner of the pool, so that anything that makes its way down to the bottom falls out back into the pool. But blanket weed of course just slithers back in, so that technique doesn't work.

The point of growing plants to reduce nitrates isn't that you leave them in there - you periodically heave out armfuls taking the nitrates with you. So seeking to control blanket weed by using fast growing plants still leaves you with lots of clear water.

That said, newts alter the scenario completely - the adults lay their eggs in broad-leaved pond plants - in my case, in the water forget-me-not and then the babies hang out in whatever weed you want to get rid of.

In the indoor tank scenario, most of the algicides also kill the snails - one of the reasons I've never looked at algicides for the outside pond.