When I started keeping tanks in the early 80's all I had was my two eyes and a small bit of common sense and a fair amount of luck.
Plants need CO2, water, light, and food.
That was the easy part.
You guessed at what ferts they needed and kept journals.
You made mistakes and corrections every few weeks.
Also, at least for me... I kept plants that I knew would grow easily.
We also had low light to medium light tanks back then because having 100+ PAR was impossible.
We used the watt per gallon guesstimations and I was usually around 2 watt per gallon.
At that light, ferts were minimal.
Aren't we all just living in a giant fish bowl?