I think it's better to start with more plants, especially ones that use alot of nutrients like water sprite or even something like ambulia if you could find it. And the ferts all list dosing amounts on their bottles. Or with a 55g you might want to look into dry ferts which would only cost $20 and would last a couple years or more for that size tank. Again much cheaper. And with the tank being low light you would only dose 1/2 or 1/4 the amount and if you ever get higher light you can just increase your ferts daily. I'll all for being economical.
Liquid carbon I suggest is Glutaraldehyde Cold Sterilization Solution 14 day 1 Gallon: Amazon.com: Industrial & Scientific. You take this gallon of Glutaraldehyde and mix it with 1-1/2 gallons of RO or DI water to make 2-1/2 gallons of Glutaraldehyde for only $27. Its the same as Excel and the same strenght only much cheaper. You have to throw away the little "activator" bottle that comes taped to the gallon of Glut as you never want to use it and the Glut has to be stored in complete light blocking bottles as light breaks down the Glut and makes it usless.
