From a mathematical perspective this makes sense as long as you have a tank where the nitrate produced from the fish waste is the same as the uptake of the plants. Seems like you would need to be very consistent in that plant mass to be able to make sure you are neither accumulating more nitrates than assumed from the fish waste or have decreasing nitrate levels over the week due to uptake. I dose the entire tank to 20 PPM because it's so lightly stocked that 20 PPM nitrates are depleted over the course of a week. You could look at it as a 40 ppm water change dose (since I'm usually changing around 50-60%) but the accumulation is functionally zero. At the end of the day I think the selling point of EI is that it's just an estimate. It seems futile to try and figure out precise amounts.
I'm only measuring nitrates though. I do wonder if I'm accidentally accumulating certain nutrients that I'm not testing for.
I get what you are saying. You do need to take into account the fish load and plant mass for each tank.
And your dosing is interesting. You are saying you dose the entire tank to 20 ppm NO3, and at the end of a week it measures zero? If so, that is really something. Most tanks create some NO3 regardless of fish stocking. I know in my tank many plants (not all) would suffer when the water column got so low.
There has been much discussion over the years about the "optimal" level of ferts for plant uptake. That is saying if plant uptake is 3 ppm per day, could you just dose 3 ppm per day and keep plants happy? Could the plants extract enough for their needs with so little in the water column? The general consensus is no. Or is there an optimal level for plant uptake? A level that allows for plants to fully uptake all they need. Many think so. For instance, in my tank, it seems my optimal level is about 30 ppm NO3. But again, that has to be taken in the context of my particular tank, with my particular plant mass and mix of plants.
Now I wasn't going to get into fish waste/plant uptake, but it looks like we are going there. In the end it's still just math.
Let's take your tank for example. Front loading 20 ppm NO3 with no fish waste and weekly uptake of 20 ppm NO3.
It would look something like this..................
Now lets say you had a moderately stocked tank producing 14 ppm NO3 per week. Here's what that would look like.
Notice the NO3 level on Day 7 of week 11. Went from about zero to 28 ppm.
Now let's say we have a heavily stocked tank that generates 28 ppm NO3 per week. Here's how that would look.
Note the NO3 level on Day 7 of week 11. NO3 is now at 56 ppm.
So rather than looking at it as trying to get to a precise amount, I look at it as many times folks don't understand or know the broad range of what is going on in their tank. IMO, it does help to understand your tank and in general what is going on, and could very well have a bearing on how much and when you dose.
And I get what you are saying about EI being an estimate. But to me it's really a very broad starting point, and blindly following EI is not optimal for many tanks.
For instance EI would have me dosing NO3/PO4/K at 22.5/3.9/22.5.
My dosing is 10/3.5/15.
And it would have me dosing micros at 2 ppm Fe. My actual dosing is 0.525 Fe.
Just saying I follow a lot of great tanks (yours included!
), and very, very few are dosing stated EI levels. They are dosing what works best in their tank.
P.S. If anyone wants to mess around with my spreadsheet calculator that includes fish waste/plant uptake, the link is in my signature below. Anyone can download it and use it however they wish.