every CM of fish needs one litre of water, that is the recommendation professionals in the aquarium business give. did your aquaium salesmen not tell yo u this?
i cannot tell you if you are 'over stocking', but always keep in mind that some fish grow to a rather large size.
i cannot recommend using any chemicals in your aquarium, this includes Easy Balance and every single other type of artificial aquarium water enhancement. the only thing i use, besides Aquarium plant neutrients, is Bactozym, which enables an quick aquarium start. i've not had any negative experiences with this but that being said, i've only used it 4 times. 2 times where i've begun an aquarium, once where i wasn't sure if i had to use it, and once where i made a major water change. i am aware that the majority of users here recommend water testers and excessive use of chemicals, but i do not recommend such.
additionally, 20% water chance once a week seems more frequent than me. i supose it's because your fish specimins need much more clean water. I change ~30% water every 2-3 weeks, and i clean my filter every 2-3 weeks aswell, but never at the same time. (i.e week one, change water, week two, clean filter, week four, clean water. etc)