Fertilizing aquarium plants and water changes
By Aram Schneider Published Updated
Translated from German with AI assistance and reviewed by our editors.
In our basics series Fertilizing a planted aquarium, part 1, 2 and 3, we have already provided a lot of information about the right fertilization of aquarium plants. Aren't too many nutrients removed that way? Do I have to fertilize my fresh water? We answer all these questions in this article.
Why change water at all?
We have already covered the topic of water changes in a separate article (see here). In short: while many natural waters have a constant supply of fresh water, in the aquarium ecosystem humans have to help out and regularly replace part of the water. This removes water-polluting elements and adds new, unpolluted water. This includes not only end products of organic waste such as nitrate and phosphate, but also, for example, proteins that hardly break down in the aquarium, if at all, and harmful microorganisms (pathogenic germs). A water change also influences the nutrient supply for aquatic plants. Certain nutrients that have accumulated in excess are reduced, and some missing or lacking elements can be added again through the fresh water.
With a planted aquarium, the question is somewhat in the air: why should you pour expensive fertilizer into the water only to remove it again with the water change?
Of course, a water change also removes nutrients. Speaking of a waste of fertilizer here would be a one-sided view of things, however. A water change also has advantages: the fresh water supply alone revitalizes the aquarium with unpolluted water; as already mentioned, organic contaminants are removed and the germ load is reduced. In terms of nutrient balance, a water change can also be understood as a kind of reset that removes nutrient spikes and restores moderate conditions. In practice, there are basically two fundamentally different approaches to handling fertilization and water changes. If you want to be economical with your fertilizers, you can determine the water values precisely by testing shortly before the water change and then calculate the fertilizer dosage so that any gaps are closed and hardly any nutrient losses occur (see the chapter "The fertilization concept with fresh water fertilization" further below). If you prefer things a little more convenient and uncomplicated, with significantly less time spent, "The fertilization concept without fresh water fertilization" is recommended. We now present both variants below.
The fertilization concept with fresh water fertilization
The aim here is constant nutrient values in the aquarium. You start from the nutrient levels already present in the old water and the fresh water, and fertilize the fresh water added at the water change only as far as needed to close nutrient gaps.
First, at the end of a week, before the water change, you get a good overview of the current nutrient situation. You measure all the important nutrient parameters of the old water, such as NO3, K, Fe and PO4. You also need the nutrient values already present in the source water, usually tap water, which can then be looked up in the drinking water analysis of the local water supplier. The target values of the nutrient concentrations that the fresh water should have after the water change must also be known. And you need to know how much old water is to be replaced with fresh water.
From these starting values, you then determine the amounts of the various fertilizers that you have to add to the fresh water to reach the respective target values of the nutrient levels after the water change. Put simply, the volume and nutrient content of the remaining water and the fresh water are divided by the total volume of the aquarium and then subtracted from the target value of the nutrient concentration. The difference is the amount by which we have to fertilize the fresh water. This gives the following formula:
[Target value of nutrient concentration in mg/l] - ([Volume of remaining water in l x concentration of remaining water in mg/l] + [Volume of replacement water in l x concentration of replacement water in mg/l]) / [Total aquarium volume in l] = [required fertilization value in mg/l]
To then calculate the exact fertilizer amount, the Flowgrow nutrient calculator can be used. With the calculator, you can determine the required amount of liquid fertilizer in ml to match a certain nutrient level in mg/l and the amount of fresh water in liters.
An example: you have a 100 liter aquarium in which a 50% water change (50 liters) is carried out every week. The measured nitrate value before the water change is 5 mg/l NO3, so that much nitrate is already present. The source water has a concentration of 3 mg/l NO3. As the target value for the aquarium water, we want to aim for a level of 15 mg/l NO3. We now calculate 15 - (50 x 5 + 50 x 3) / 100 = 11 mg/l NO3. Our aquarium of 100 liters therefore needs fertilizing by a total of 11 mg/l NO3. Now the Flowgrow nutrient calculator comes into play. At the top under "Tank volume", we enter the amount of aquarium water, in this case 100 liters. Further down, under the nutrient nitrate, we first pick a suitable product with which we want to raise NO3. In our example, we use GH Boost N.
Now you have to experiment a little: under dosage, you enter a number and then click "Calculate". Using trial and error, we then have to find out the dosage until our desired fertilization value is displayed under nitrate in the results below. In our example, this was 11 mg/l NO3, which we reach with an approximate addition of 15 ml of GH Boost N.
If the nutrient level of an element in the aquarium water is already below the target value, either the daily fertilization has to be adjusted or a further calculation for additional fertilization of the old water has to be made.
Quite complicated, isn't it? This way of dealing with nutrient values and the water change is very meticulous, but with the weekly testing and calculating it also involves a certain amount of time. We would now like to present a somewhat more convenient approach.
The fertilization concept without fresh water fertilization
The idea behind this concept is basically based on the Estimative Index method. It provides for unlimited nutrient availability. Since all nutrient elements are present in lavish amounts, they don't have to be checked regularly with water tests, as a nutrient shortage generally doesn't occur. A regular weekly water change of 50% ensures that nutrients don't accumulate too much. Nutrient spikes are simply cut off again by the water change. It is therefore intended that nutrient values are low at the beginning of a week and rise toward the end of the week.
This basic idea is adopted here in principle. The daily doses of the fertilizers are, so to speak, always a little larger than the plants' daily nutrient requirement, so that certain nutrients accumulate over the course of a week. With the water change at the end of the week, you then carry out a kind of reset, cut off any nutrient spikes and set them back to a minimum or to a certain baseline value.
In other words: by slightly overdosing the fertilizers over the course of the week, the fertilization of the fresh water is, so to speak, already included. This eliminates the weekly final check and the regular testing of the water.
The amount of fertilizer added over the week depends on the size of the water change. The more fresh water is added at the change, the more the daily fertilization should be adjusted slightly upward. We definitely recommend a water change rate of 50% per week. That way, the water change still has enough influence on the nutrient balance and on other water pollution (germ density), but on the other hand does not remove too many nutrients.
Of course, to find the right daily doses, you have to check the water values somewhat more often with a suitable test at the beginning. At some point, however, you reach the point where daily fertilization, the plant stock in the aquarium and the weekly water change form a certain balance and all values stay within the desired target ranges. Instead of regular testing, you can then rely more on simply observing the plants. Only when deficiency symptoms or algae growth appear is it worth switching back to "analytical mode" and checking certain parameters with water tests as well. All in all, this method without fresh water fertilization is the much less complicated one, but sometimes requires a certain amount of experience.
Are you having problems with your plant growth or your fertilization concept? No problem, write to us and we'll be happy to help!