Macro Dosing: Stop asking “How much?”

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Spending time in aquatic plant forums, and watching people continuously ask “how much” with regard to macro dosing for a particular tank drives me nuts! People want to know the quantity of a fert that they should dose their tank. It doesn’t make me nuts so much because it is a bad question, but because so often it is the wrong question. And a very clear tradition exists in this hobby to answer all dosing questions by describing how much to dose. Problem is - of course - that it is usually a meaningless answer.

When someone describes a quantity of a macro fertilizer to dose, it ignores plant uptake rates and water replenishment rates. To say that your tank needs X grams (or teaspoons, or what ever measure you feel happy with) of KNO3 every other day completely ignores water change intervals. I’ve got discus, so I change water more frequently than is normal in the hobby. So while X grams may be enough for the average hobbyiest for a tank with size, plants and lights like mine, I’m going to need something more than X, because I’m flushing so much down the drain so frequently. Likewise, the same recommendation of X grams KNO3 completely ignores plant uptake rates. Some plants have dormancy periods, recent trimmings or placement in a tank will be a factor, as will the general health of the plants, not to mention light, CO2 or other limiting factors. Different tanks of the same exact size with identical lighting can absorb any given nutrient at wildly differing rates.

So to reccommend X amount of any nutrient does a disservice to someone needing to know how much to fertilize their tank. But there are important exceptions to this.

If you are dosing Tom Barr’s Estimative Index (EI), or any other method that ignores day-to-day nutrient levels within a tank, please ignore every thing I’ve said. EI is founded on the idea that you establish an amount - “X” if you will - of every needed nutrient that will be at least enough, if not more than enough, for the plants. Then you dose that amount regularly - knowing that your plants will alway have enough - though not knowing if they might have too much. Frequent and regular water changes insures that any nutrient buildups do not become extreme.

So if you are not dosing EI or a similar method, what should we ask with regard to dosing, if not “how much”?

We should ask “what’s my target level“!

Concentrations of nutrients in the water column - expressed in some meaningful way like Parts Per Million (ppm) - is the ideal way to communicate dosing requirements. And I say target level, because short of some form of perfect constant dosing mechanism, soon after you dose anything, some of it may already be consumed by your plants. You could have a ppm target, and even if you dose perfectly, that level is going to change soon. Let’s say you target 14 ppm for Nitrogen. Depending on your plants’ uptake rate the nitrates in your tank may fall to 12, 10, or even 0 (that would be hard though) by the end of the day. So if you knew that your tank’s uptake rate for nitrates was 4 ppm per day, you might target 14 ppm by dosing enough to make the tank 16 ppm. So the tank starts the day at 16 ppm but falls to 12 ppm over the course of the day. Your average worked out to 14 ppm - the level you were targeting.

Dosing to target levels requires a good understanding of your plants’ uptake rates. And that requires test kits. If you don’t want to deal with test kits, then you need an “ignore the level” method like EI. Go take a look at Tom Barr’s website and read all about it. Otherwise testing is required. And good record keeping too if you are ever going to understand your plants’ uptake rate. And it’s easy to understand those rates. Test after dosing, but before the lights come on, and test when the lights go off. The second test should show lower ppm’s. The difference is your daily uptake rate. If nitrates started the day at 16 ppm, and ended the day at 12 ppm, then 4 ppm is indeed your tank’s daily uptake rate for nitrates.

Once you know your uptake rates, and you know what level you want to target (do you want 14 ppm for nitrates?), then you can go figure out how much to dose. There are great websites out there that can help you with that. Fertilator on Aquatic Plant Central is one, and Chuck Gadd’s website is another. If you know what level (in ppm) a nutrient is in your tank, and you know what level you want it to be, these sites will calculate the “how much” question for you.

At this point some of you may be asking “But how do I find out what my target levels should be?” Well, that’s the 64 thousand dollar question, and I’ll not try to answer that here. But what I do hope, is when you get onto a forum or go to a web site to find out, please don’t ask “How much of X nutrient do I put in my tank?”

Ask “What should my target level for X nutrient be?” That will get you the answers that you really need.

And some of you might be thinking… “That’s fine for Nitrates and Phosphate, it’s easy to get cheap test kits for those. But not Potassium.” That’s true. Potassium kits are rather expensive. But once you know your uptake rate for Nitrates, you can assume that Potassium will be consumed at roughly the same rate. If it turns out to be more, and you aren’t dosing enough, you’ll see pin holes in the leaves of your plants. And if you are actually dosing a little too much, that’s fine for potassium - assuming you are doing regular water changes to keep the excess from getting out of control.

And some of you might also be thinking… “But why should I care about this?” Easy. I’ve already shown that asking “How much” leads to not really understanding the macro concentrations in your tank. And too little will starve your plants and algae will soon follow. And dosing too much will likewise lead to algae. Yes - I know many will say “EI” provides too much, and that doesn’t lead to algae. True. For some people in some tanks it doesn’t. But it does not work for everyone. I’m not sure why. I believe Tom Barr’s position on that is that if excess is leading to algae then something else is wrong, like not enough CO2. But I don’t believe that is always the case when EI does not work for someone. There are a whole lot of experienced, clever people out there that EI has not worked for. So I’m gonna stick with my belief that unless EI does work for you (and it just might), then too may nitrates or phosphates may lead to algae trouble in your planted tank.

On a final note… please notice that this entire rant is about not asking “how much” for macros. Not micros. Micro-nutrients are not easily managed by testing your water (yes - I’m including iron in that statement). For micros you have to ask “How much”, because you cannot reliably know concentrations or ppms.

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