) Hi, I really have no idea how many gallons/litres my tanks holds, it is 40 inches tall and 40 inches wide, but also hexagonal, so it would loose a little due to the shape, what would educated guess be?
Also I have 4 Mollies in the tank (inch in size each), how many fish should I have in there? I want to add 2 more very small fish can I and which ones?
A tank measuring 40 inches by 40 inches would not be a mere 5 gallon affair. Either you have got your units of measurement wrong or they are thinking of the wrong tank. Did it not have a box or spec sheet? There are plenty of websites that do imp/dec/US conversions. Google it.
These are dimensions of All Glass's hexagon aquariums. Perhaps you will see one similar to yours. This is a US manufacturer and the measurements are in inches and US gallons:
It would be helpful if you put you location in your profile. The members would then have a better idea of what you are talking about. This is particularly important if you ever seek advice about sick fish and need a medication recommended. Beside that, it's fun to know where everyone is.