Warning: fertilizer ramblings. Skip if you don't care about numbers!
A few weeks of 30-9-39 and I'm hooked, maybe for life. 70% Water changes, with 20-6-26 initial recovery dose, then a 10-3-13 booster dose midweek has created the single most algae-free, fast growing, healthy tank I've maybe ever had. The pearling has reached "upside-down rain" levels daily. The BDBS remains clean and algae-free, too.
How I've been fertilizing:
I prepare 4-5 weeks worth of
GH remineralizer, initial doses, and booster doses
in little 2oz cups.
I have been doing it this way because It's nicer to sit and spend 30-40 minutes making 5 weeks of doses in a single go, than to get the items out every weekend and measure/dose manually. It was also nice to "hold off" on making these doses if I wanted to change or experiment, but I'm ready to just make a Macros solution at this ratio (30-9-39) to make dosing easier. Remember, part of what matters when determining your fertilizer concentration (like 30-9-39) is the water change %. If you only perform 30% WC each week, 30-9-39 will quickly reach toxic/unwanted levels in your aquarium. Conversely, if you do 70% WC each week, you get a lot more leeway to experiment, but will have to dose as much as I do, or similar, to prevent your nutrients from bottoming-out. If you want to understand this or explore more, check out my
Nutrient Accumulation Calculator!
Making a Macros Solution from scratch:
The first limiting obstacle is K2SO4, because it is the least soluble out of all 3 macros. If I add too much, it will never fully dissolve. So, I'll figure this one out first and go from there.
When you add enough KNO3 per week to get 30ppm
NO3, you also inherently add 18.9ppm
K. That's because it is
KNO3. It contains both macros.
When you add enough KH2PO4 per week to get 9ppm
PO4, you also inherently add 3.7ppm K (total 22.6ppm K from these two macros already).
I found these numbers through
Rotala Butterfly.
That means I need to add enough K2SO4 to raise the final amount of K by 16.4ppm, to reach a total of 39ppm from all 3 macros per dose/week.
So, I need a solution with "reasonable dose sizes" that has K2SO4, that raises my tank's K by 16.4ppm. Since a mason jar will easily hold 600mL, let's use 600mL as the base for this solution (I love mason jars!):
For K2SO4:
Ok, so using the calculator, I've determined that I'll need 47g K2SO4 in 600mL to have a solution that, when 60mL are added to my 34 gal aquarium, I raise K by 16.4ppm. Great!
Side note, what happens if you make a solution too concentrated with K2SO4? You'll get a warning:
RotalaButterfly claims that the solubility of K at room temp is 120g/L (0.12g/mL), but that's the solubility at 77F... I don't know anyone who considers 77F room temp?! My room temp is much cooler, especially during the winter, and I find that K2SO$ will precipitate out (not fully dissolve) if I push it too close to this solubility limit in these cooler temps. Also, when making a macro solution with KNO3 and KH2PO4, which both add K, perhaps
that affects solubility as well. Either way, I prefer to play it a bit safe near 80% solubility limit, instead of at 100%, so I can be sure all the K2SO4 will dissolve. And I tested it; K2SO4
does fully dissolve, even with the other macros present, at this concentration (0.08g/mL) at room temp or even cooler (67F).
Anyway, again, I now know I need to add 47g K2SO4 to my 600mL of water.
Now that I have K2SO4 out of the way, I don't have to worry much about the other two macros.
Both KNO3 and KH2PO4, at the levels we use them, are WAY more soluble than K2SO4 and they won't be a problem:
For KNO3:
For KH2PO4:
So, to make a macros solution where dosing 60mL into my 34 gallon aquarium raises the NPK by 30-9-39, Here's the "recipe":
In 600mL pure RO water, add:
- 63g KNO3
- 16.6g KH2PO4
- 47g K2SO4
- 0.25g Potassium Sorbate (anti-mold preservative)
- 0.25g Ascorbic Acid (lowers pH, helps anti-mold preservative work)
Once fully dissolved, I can add 60mL of this solution to increase my 34 gallon water column
ppm to 30-9-39.
HOWEVER, since I have been LOVING the "mathematical consistency" of large water changes with large recovery doses, and a midweek booster dose, I will do
two doses instead of one:
- After the weekly 75% water change on Sunday, I'll dose 40mL which adds 20-6-26 ppm NPK.
- On Thursday morning, I'll dose 20mL of the solution, which adds 10-3-13 ppm NPK.
And that's it!
Rather than measuring
2-3 different dry powders, betwteen
fifteen cups (35+ measurements! on the scale!) every 4-5 weeks, this solution requires only measuring 6 ingredients on my scales (water, KSorbate, AAcid, KNO3, KH2PO4, and K2SO4) once and lasts for over 10 weeks. It's also already dissolved and easily dosed with a 20mL syringe and stainless steel needle.
Summary:
- KNO3 and KH2PO4 already add K on their own; discover this ppm already added, and subtract that from your desired K amount. That tells you how much K2SO4 you need to add to reach your total ppm K concentration in your solution.
- When making a Macro solution with KNO3, KH2PO4, and K2SO4, make sure you can fully dissolve all the K2SO4 you plan on using in your solution.
- Add them all together with a bit of Potassium Sorbate and Ascorbic Acid (or vinegar!) in pure RO water, and dose according to your solution's concentration relative to your tank's volume.
I know none of this is revolutionary, I'm just typing it out in case it helps someone understand how to do this themselves since I was taking the time to make a solution anyway.
super sharp, it is literally sandblasting grit 
and had burrowing snails stressed to the point of death.. but I have not personally had damaged loaches or cory cats!
. It's clear from this forum, there are far bigger aquarium nerds than me. 
