trial and error by undersizing the aquarium and overstocking it, only to have your stock die and find out it's not gonna work, then RESTOCK it? are you kidding me? that isn't how you learn. that's how you repeat being an idiot and causing more fish to die because you aren't being responsible.
if ever there needed to be a license to own pets, have a kid, etc, YOU are the perfect reason why. just because they're "cheap" doesn't mean you should be able to just continually kill them off. get another hobby if you can't accept that they have requirements that need to be met.