What's Wrong With My Tetra?

Following on from Sue's good advice, your GH or dH of 10 (assuming it's a reliable test you've used - test strips are notoriously inaccurate on this, for example) equates to 178 ppm.  In that case, your water would actually be ok (ie not too hard) for neon tetras.  However, your water is actually too soft for mollies which might account for its early demise, and you may well find the others follow suit as inappropriate water affects their immune system and in turn can result in premature disease/death and them not fulfilling their life expectancy - I certainly wouldn't get any more of them.

I don't know where you have looked but the site with the most reliable information gives the hardness range for mollies as 15 to 30 dH. Mollies do suffer more than most hard water fish if the hardness is not high enough, often suffering from something called 'the shimmies'.