How do i acclimatise nerite snails ???

Float them...add a little water and drop them in. Preferably onto your sand.
 
Oh I didn't even read that you wanted to make them into freshwater...sorry. I thought you asked how to acclimate them.
 
Drip them slowly. The don't just eat SW algae, pretty much any algae and diatoms in both FW and SW. Depending on the species, there are FW and SW species that are labeled nerites but many are brackish so they can go both ways.
 
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