No it doesn't affect snails, shrimp or plants.
Levamisole Hydrochloride is safe for all fish species, however if you overdose, then the fish will become stressed out. Small fish and baby fish will be stressed out first but bigger fish will also become stressed. If you notice the fish become nervous and skittish after treatment, you have overdosed and a small partial water change will fix the problem.
-------------------------
Before you treat the tank, do the following things.
Work out the volume of water in the tank:
measure length x width x height in cm.
divide by 1000.
= volume in litres.
When you measure the height, measure from the top of the substrate to the top of the water level.
There is a calculator/ converter in the "How To Tips" at the top of this page that will let you convert litres to gallons if you need it.
Remove carbon from the filter before treating or it will adsorb the medication and stop it working.
Increase aeration/ surface turbulence to maximise oxygen levels in the water.
-------------------------
You treat the fish once a week for 4 weeks. The first treatment will kill any worms in the fish. The second, third and forth treatments kill any baby worms that hatch from eggs inside the fish's digestive tract.
Treat every fish tank in the house at the same time.
You do a 75% water change and complete gravel clean 24-48 hours after treatment. Clean the filter 24 hours after treatment too.
The water changes will remove most of the medication so you don't overdose the fish the next time you treat them. The gravel cleaning will suck out any worms and eggs that have been expelled by the fish.