When should snail be added to a tank?
 

I have a 70litre tank working about 3 weeks now is it safe to add any snails?

4 danios (2x gol & 2x zebra)
6 black phantom tetra

If the fish are doing well, there is no reason you can't add snails. How are your water parameters, have you checked them with a test kit? You want to start off slow to give the tank time to cycle. What type/size of snail and how many?

I dont know what they are. Someone off freecycle is giving them away as have too many. She says they are about 5mm in size I have a 70l and 15l both have been going for 2-4 weeks and parameters are both great

too many snails often means malysian trumpet snails in my experience lol. They reproduce very rapidly. You can try it, although might want to watch and see if your fish eat them.