Hard water community, 20 gal?

[Chickadee]

I've been researching this online but need help narrowing my focus. Here is my long list: endlers, cherry barbs, pepper cory, otos. Guppies, maybe. Not really interested in goldfish or platys. Worried the water will be too hard for mystery snails. Is the nerite a better choice? Other suggestions? I want peaceful fish that would hopefully leave snails alone. Am I asking for too much? What concessions will I have to make with my water hardness and tank size?

 

[Essjay]

Remind us please - how hard is 'hard'?

 

[Chickadee]

Remind us please - how hard is 'hard'?

On the strips I was using, it's at the highest reading (purple) 180 ppm, but I was told it could be even higher than that. I had a hard time finding the liquid test and finally found it at Walmart, of all places. Should be here in a few days and then I can get an accurate reading.