Embrace, I'm sorry your hobby has started off so badly
How did you acclimitise the fish to your tank water, including a guess at the length of time?
Do you know how long these fish had been at the store before you bought them? Weak fish that struggle with the transportation to the shop often die in the first week or so of arriving.
Did you check with the store about the water they were using (i.e. did it have the same gH and kH hardness as your tank water; similar pH; temperature)?
Can you take a 25-30ml sample of water to your fish shop for ammonia and nitrite verification, especially if you are using strip tests?
Did you de-chlorinator added to the right concentration for the water you added just before getting the fish? My Nutrafin suggests 5-10ml per 8 imperial gallons.
I'm sure there are other questions that more experienced keepers will add to the above.
When I saw your thread in "tropical discussion" alarm bells started ringing straight away with me about adding so many fish to a 60l tank, bring it to or very close to maximum safe stocking levels, in one hit. I'm clueless about the hardyness of guppies, but I would said that adding the six Cherry Barbs would have been enough for the first fortnight; then the Gourami; the guppies(?);
5+ corys; the Cardinals in fortnightly purchases... providing everything was working out well.
Cardinals and Neon Tetras, despite what is often said by fish shops, are very fragile fish. They handle transportation badly, not to mention changes in water vitals and they have bad long-term prognosis in harder water (kidney failure I believe). For instance, "markandhisfish" on here recently set up a Discus tank and wanted a huge schoal of tetras, I think Cardinals. The first batch of ~20 all settled in well, but then he bought a second batch of ~15, which all sadly died over the space of about a fortnight. It came to light that the second batch were only in the shop for a few days before he bought them.
Finger crossed the others pull through and some more experienced keepers "pipe in" and give their two pennies worth...
