any aquarists here? I need some help...

[Crispy42]

With the high nitrates, cloudiness, and the recent addition of the snails, it sounds like your tank is going through a mini nitrogen cycle. The addition of the snails (mystery snails are HUGE waste factories and are really too much for a 10 gallon to handle long-term) added too much poop and resulting ammonia for your established beneficial bacteria to handle. If your tetras were old or weakened at all they could easily have died because of this ammonia spike. The bacteria eventually would have converted it to nitrates, but cloudiness in the tank (unless it's green algae clouds) is a classic sign of the nitrogen cycle.

Its always possible that that the snails had brought home an illness from the pet store if they weren't quarantined first, but it is extremely unlikely that your fish would all die so quickly without showing symptoms first.

How often do you perform water changes? What percent of the water do you change each time? Did you also change the filter cartridge at the same time?

For now you will need to do daily partial water changes (up to 50% at a time, but no more or you risk killing beneficial bacteria) to keep ammonia/nitrite/nitrate levels safe. Also, I hate to tell you but really neither the snails nor the Pleco are suitable for a ten gallon tank. I wouldn't add any new fish while your tank is cycling. Wait until it is stable before adding any new fish.