I suppose I'm going out on a limb with this post. However, I think it's necessary for others to grasp the idea behind fertilizing our tanks.
Plants will be limited by something. There's no way around it. Provide an absolutely perfect environment and the growth will still be limited by the plants themselves. i.e. You can't expect ten feet of growth per day.
So where do we draw the line? Do we limit growth by light, CO2 or fertilizers?
In my opinion the last thing that needs to be limited are nutrients! It's the easiest of all the three three things plants expect. Sure there's more than those three aspects but those are the big three in my opinion. Everything else seems to be geared towards controlling one of those three core concepts.
Which method is best?
That's always been, and probably always will be, a heated debate. The fact is, plants could care less! Provide nutrients, end of story. The problem arises when we want to do little to no water changes. Now we need to rely on testing the nutrient levels to see what is not only being used but produced or added to the tank. The nutrient balancing act now becomes very tedious.
In your original post you say you intend to do a 50% water change per week. That's great! This means you really have no need to worry about water testing. This is essentially the idea behind the EI method as I understand it.
Will the plants need full EI dosing without CO2 and high light? Not at all! In fact maybe 10% of full EI dosing. However, the important question is, how close to nutrient limitation do you want to come?
You can safely dose full EI IF
you're doing 50% weekly water changes in a non CO2 tank with low light. Heck, you can dose that to a tank in your dark closet without CO2! The nutrients won't be the issue! The key is the water changes. Those water changes keep all levels within range despite plant uptake. There is no need to tinker with fertilizing provided you do the water changes you said you will be doing. Reduce the dose to say 50% EI? I would say that's a safe assumption. Do you really care about twenty five cents a week in fertilizers if you don't? Seriously, this is very cheap stuff.
Hoppy brought up a good point about water changes and CO2 levels. Tap water typically has high concentrations of CO2. If we have fluctuating CO2 levels plants can have problems and algae can be signaled to grow primarily BBA. Will this happen with a weekly water change? The jury is still out on that one. I've personally never seen a big issue with it. I think the good out weighs the bad in this respect. To avoid the problem entirely aerate the water overnight before adding it to you tank. Now the water is in equilibrium to the tank water.
In a nutshell what I am saying is to dose your tank with maybe half EI and move on. Do you water changes and enjoy the hobby. The fertilizing will take care of itself.
btw, yes I know this was a very old post