Any molluscide you use is likely to kill fish as well, but even if you found one that killed the snails and the fish didn't die, the water will become toxic because of the presence of so many dead bodies.
The usual means of preventing such an infestation is to quarantine plants before placing in your pond, to check for any kind of pest such as this, or 'sterilising' the plant first with a solution of permanganate. Either that or buying it from a reputable supplier. However, now that you've got a problem, it might be that the only answer is to contain the fish and plants in something else, empty and thoroughly clean the pond out, refill and replace the fish - but don't replace the lotus plant without using permanganate solution first. Even this might not be successful, but is probably worth a try.
Some caution is necessary when handling the snails and water - water snails may carry parasites which affect humans (bilharzia is the most well known, but not sure about your area). Snails do serve a useful purpose if you can keep the numbers down - they eat algae which builds up in ponds.