Please again do NOT take this as an attack on you.
It doesn't matter whether the tank is working now or not, I can fit in a garbage can...long story but amusing pictures...would I be able to live in it - no. I would get diseases with no toilet or my own space to comfortably move around in....this is what your fish tank is like, only your fish cannot tell you.
They can though show you that they are not happy either through injury, disease or in worst case death. Pop eye, ich, any fish disease or injury is that indicator.
I found a site that you will probably be able to order the API test kit from..
, it might be worth checking to see.
If you can absolutely get a new tank fairly soon ie within 2-3 months, then I would say (very reluctantly) keep the fish..but you would have to perform daily BIG water changes and not overfeed. This is a last last resort if you really cannot take the fish back or give to them to someone with a more suitable tank.
We are trying to help as much as you can, treating for pop eye is going to involve medication, which in such a small tank can be bad for the other fish if they are not showing anything.
I have no say what YOU as a person do, that is your decision and yours only. What we (as a community you joined) have done is offer advice to help you enjoy your fish without having to constantly battle diseases, injury or deaths.
If we can offer you anymore help / assistance to make this work out then we will.
I know this may seem like a weird question, but do you have anywhere you can get a large / long plastic box from?