I think you hesitancy to put a snail into the aquarium is commendable. When Dr. Harrison suggested that to you, I think he was assuming that you were familiar with what conditioning and seasoning of the water is and familiar with cycling an aquarium. Unfortunately there are stores which don't tell customers about that, but don't let their ignorence and irresponsibility cripple you in the hobby.

Do you understand what cycling an aquarium is all about? Down below you ask if you should put more water conditioner in the tank? Do you have an idea of what kind of water conditioner your should use? Again, what has your municipal water department put into the tap water which you would need to treat for. Are you aware of the fact that a conditioner will not cycle an aquarium? You may want to read several of the sections of the Immediate Help section.

Your concern about a shop putting a few relatively inexpensive fish in a tank is good. However, what is the difference between what they are doing and what you did?

That is why I have suggested that you please go to the Immediate Help section and check out what cycling an aquarium is. That is also why I suggested in a comment above that you look into that fish-less cycling and probably get test kits for ammonia, nitrites and nitrates.

If you aren't willing to at least check into what cycling is, you might just as well take that big tank and set-up back to the store and give away your guppies. Then you will have more time for the things which interest you. ;)