If you already have inverts like shrimp in the tank, then add some carbon to the filter to remove any copper medications that may have been used on the plants to kill off snails, and don't bother treating them as it would be creating a problem where there is none.
Otherwise, I really wouldn't worry about it.
Snails do no harm and are actually beneficial to aquariums, if they overpopulate (will cause no harm, but some people dont like seeing snails) the tank, it's only because they are finding enough food to do so. This is very easy to prevent: gravel clean more and/or feed your fish less food. All the snails are doing is eating algae and waste food/organic matter like dead plants. So for every snail your see in your tank, there's less waste just lying around decomposing.
Hydra are pretty cool, and pose no threat to anything but the smallest new-born shrimp and tiny egg-laying fish fry. So don't bother trying to get rid of them unless this is a breeding tank for egg-layers.
And the chances of getting anything else like any disease or parasites from the plants are so small its not even worth worrying about IMO. The vast majority of fish diseases wouldn't be able to live in the plant storage tanks without fish there.
If your still worried about anything, then use a copper treatment on the plants in a separate container for a day or two, and then I would rinse them and add carbon to the filter of the fish tank before putting them in (even if you don't keep shrimp, I would still do it as copper isnt good for fish, even if it doesnt kill them it cant be good...).
