Hello! Glad to see you here
But, I am going to be the bearer of some bad news.
First. A 1.5g tank is farrrr too small to house anything except a betta - and even that is questionable.
Secondly, the fish you have are all schooling fish. That means that they need to be kept in schools, or groups. Otherwise, they become stressed and will get sick. You would need to have 6 or so scissortails, as well as 6 neons and black neons.
That said, I think you may want to consider taking back your fish for the time being. You should read up on what it means to cycle a tank (fish poop creates wastes which build up into toxins that are very harmful to fish and can and will kill them. There are several threads on it here, as well as stickies).
Get the biggest tank that you can afford. I wouldn't go any smaller than a 10g tank. But bigger is better (like a 29g). They are easier to maintain and have the possibility to house more fish. In a 10g tank, after it was cycled, you could have about 8-10 of those neon tetras in there, or 6 or so scissortails, or 6-8 black neons. Or you could get a different species of fish such as corydoras species or danios or somethign along that line.
Walmart carries 10g tanks that come with the lights, cover, heater, filter and net and some trial fish food and stuff for 50$. A 29g kit is 150$.
Fish also need their water to be dechlorinated because of the chlorine and chloramine that we put into our drinking water. They sell bottles of the stuff for little money at any pet store. Make sure it gets rid of clorine AND chloramines. They are two different things.
Also, the fish you have are tropical, which means they need their water warm and require a heater. You can't really use a heater in such a small tank as you have now, which is another reason it wont work in the long term.
These fish also require a filter (neons are particularly noted for being fickle and generally "not hardy." Scissortails and black neons aren't far off). The filter will help faciliate cycling the tank and making the water clean for the fish to live in.
One of the biggest pieces of advice you will learn here is NOT to trust the employees at the pet store. As a rule, they have no idea what they are talking about (as evidenced to you by the recommendation of 4 fish per gallon - which is completely arbitrary and I have no idea how they'd come up with such a thing. I mean, fish don't grow to be all the same size!). Take their advice with a (large) grain of salt, then return here and post your question to us or ask us what we think about their advice
There is soo much to learn in this hobby, but luckily we're all here to help!
EDITED
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