You could just start with the 20g formula once a week and see how it goes. there are too many variables for us to tell you exactly. For instance we have no idea how many plants you have or what the substrate is or what your tap water is like. What plants are you trying to grow?
Good idea, and I was thinking of just using the 20g formula. Since I have a 37 gallon tank, which is close to 40, I was thinking about using close to twice of that formula. As far as substrate goes, the one that I have at the moment does not provide any nutrients. When it comes to plants, I have Trident Java Fern, Java Fern, Philippine Java Fern, Java Moss, Anubia, and Jungle Val. Which information about my tap water would you need? All I know about my tap water is that the PH is at 7.6
I don't want to sound dumb about my following question. Is the substrate only important when you have plants that are rooted? What about if I didn't have the Jungle Val plant, would a substrate with nutrients still be needed for my setup?
I almost forgot to say, but I think my tank is heavily planted.
Fertilizing doesn't need to be done accurately. As long as you dose enough so none of the nutrients is in short supply, thus limiting the growth rate of the plants, you are ok. However, if you dose more than the plants are using, the excess will eventually build up in the water to the point where it is harmful to the fish, and eventually to the plants. That doesn't happen in a week or two, but it can eventually happen. To avoid this just replace about half the water in the tank every week or two. That removes enough of the excess to prevent it ever being a problem. In a nutshell that is the Estimative Index method of dosing fertilizers.
Thank you for your reply. Someone told me that having too much iron could be bad for the fish, is that true? Is there some type of website that gives an ideal of how much ppm I need for Macronutrients and Micronutrients?