Planaria might attack shrimps but attacking and eating ramshorns seem impossible.Maybe they are very old already?Or changes in parameters.I wouldnt exactly worry about ramshorns though as they breed easily and stuff.
In my experience, too much of them do decline the Ramshorn snail and shrimp population over time.
How many is too much is hard to say, it's said 99% lives hidden in the substrate. Then if you spot 1 it's a 100 more you don't see. You can use a Planaria trap with some bloodworms in it, or a small piece of liver or Beefheart they are especially attracted to anything bloody. If you catch more than 5 overnight, you might have a pretty large planaria population dwelling in the dark.
But this can also kill off some snail species as well. As for example MTS doesn't like it very much but do survive it. I guess the snails tugged well away in the substrate are less affected by this stuff. Pond and Ramshorn are not at all affected by it.
There are some dewormers available from the vet that are very effective too that do not seem to affect snails..
Since you are only killing living worms and not the eggs it is about impossible to get rid of them completely. But it does maintain the population.
Yes if you have a very large planaria population it will result in a lot of dead worms in the substrate that can lead to ammonia spikes in or near the substrate.
I forgot to mention if you have a healthy Ramshorn population then crush a small Ramshorn and put that in the planaria trap. Ramshorn meat also attracts Planaria very quickly. Doing that every day might keep both populations well enough in check.