First off, I'm no pro, and I've only been doing ferts for 3 weeks... but hey, it's free advice.
Basically, from what I gathered, how much you dose will depend on how much light you have, how much CO2, and if your soil has high CEC (will uptake the nutrients and leach them back out to the plants).
My tank is a 12 gallon with medium light (2.25 w/gal), I'm doing DIY CO2, and I have ada aqua soil. I was reading this thread
, in which Tom Barr (plantbrain) suggested that with lower light and good soil, you can dose at 1/2 EI with good results. That's what I'm trying for now, and I'm seeing really good growth so far from all my plants.
I'm used this article as a reference for EI: Estimative Index Fertilization Method
I picked up a pound of each KH2PO4, KNO3, K2SO4, and Plantex CSM+B from GLA
for a little over $30 shipped. It should last me a couple of years. I pre-mixed solutions with distilled water such that 5mL is 1/2 of a normal EI dose. All of the macros are in one solution, and the Plantex CSM+B is in the other one (you don't want to mix those two).
There are other dosing methods to use, but I chose EI because it's super stupid easy and it follows a routine that I'm already doing. I have a 10 mL syringe that I draw half full of the fert solution, and just squirt it in. Macros one day, micros the next. I have to feed fish daily anyway, so I just do this at the same time. It adds no work.