Join Date: Nov 2018
Location: Oil City PA. USA
Ok how long has the tank been cycled, and up and running? If the tank has not been cycled, and matured (at least a month after the cycle finishes) your mystery snails may be starving. Does the tank have any algae in it? The Algae is their main food source. You can also place soft blanched veggies in the tank for a food source. Are there any live plants in the tank? This would be another source of food for them when the leaves drop to the bottom of the tank they will eat them as they decay.
So the pet store employee you talked to was right the tetra will not bother the snails. But the water conditions would. If the tank is not cycled and your water parameters are not (ammonia, 0 PPM, Nitrite 0 PPM and Nitrate between 0 and 10 PPM. The tank was not cycled and adding Snails to a tank that is not cycled with Tetra would cause a very high Bio load. (which means you have a very high ammonia reading of toxic ammonia.