I agree with F fishorama , cycling a tank takes up to 6-8 weeks, maybe bit more if something gets stuck. Lack of nitrates is usually sign of uncycled tank, I'd recommend you doing a 75% and see if he perks up.
Yes, your water is too hard for rummynose tetras, just like all the tetras, they need soft water.
When cycling a tank with fish in, you will have to test your water for ammonia (NH3/NH4) and nitrite (NO2) everyday and cut down on feeding your fish.
There is a "rule", a rule but not quite, that every user of any forum will say: 'Never trust stores, they look like they know everything about fish, but the truth is that they want to make a sale, only money matters to them, not wellbeing of fish or any other pets'
Honestly, I have heard one employee of my local store say that goldfish "will be fine" in a 30 LITRE tank, the truth is they won't be fine.
Its really sad for me to see fish suffer, because stores and most people have no real knowledge about fish.
Always go on forums, no one here will mind you asking thousands of questions. We'll always be happy to answer even the most stupidest question you have got there