The inch rules barely applies to any fish, all fish are different and require different things. I have a khuli loach about 3 inches in length in a 10 gallon, that would mean I can only have 3 of them which is absurd. You should never use it when stocking any tank. Things that need to be considered (amongst other things like water temperature) are the amount of oxygen they use (how active they are and how big their gills are), how much waste they produce, what available space they take up (where they hang out) and of course how big they grow which is why should always compare adult sized fish. A small tetra such as an ember or a neon has poops about 1-2mm big, if that, - a tiny amount compared to a gourami or any larger fish. My gourami did poops as long as a neon, even longer sometimes, i was quite astonished. The amount of oxygen they use up is also small when you compare these things to most larger fish. A fancy/fantail goldfish, in comparison produces poops about 1 inch or more in length, and many more times thickness. And the amount of space they require to swim about is huge, at least 3 foot. In a 20 gallon with all these things in mind 30 neons would take up about 90% of the space, theoretically allowing for more fish. But this would mean your tank had no decorations, gravel etc... Plus I said you shouldn't max out the capacity, for these reasons. The more fish you have the more you will have to change the water, if you can stay on top of that there is no reason why you can't have a larger shoal. Plants also help to deal with the nitrate so a good plant setup would also help. My shoal of tetras seem very happy, but I constantly care for them, check for any plant matter that needs removing etc...and change 30% of their water 1-2 times a week when required. If you don't have time to do these things then you either need a much smaller shoal (which is bad for the fish), choose a different type of fish or pick another hobby.
