Not to confuse things further, but Nitrogen (the N in NPK macronutrients) comes directly from fish waste. A few water systems have nitrates in them (well water/agricultural zones especially), but I wouldn't count on it.
For a lower light low tech tank I'd just stick with a basic microfert like Flourish comprehensive, and if you want to see if co2 will help noticeably (it may or may not depending on how much light you are working with), go with either a DIY co2 setup or start dosing excel /glutaraldehyde, which is a co2 supplement. It's often mistakenly called liquid carbon, which can confuse things even more, but either way.