Incompatible Aquarium Plants
Incompatible Aquarium Plants
How many ‘easy-to-grow’ plants have died in your aquarium? If your aquarium is anything like mine,
quite a few. Perhaps you’ve remodeled your aquarium, added new plants, rearranged others and now
find that plants that grew well before are not doing well. Why does that happen?
It might not be you. It might be those other plants. Those languishing ‘easy to grow’ plants might be
victims of the other plants.
That’s right. Not all plants get along with each other.
The ‘model’ promoted in the aquarium plant books I’ve seen is the ‘One Big Happy Aquatic Garden’
model in which they all get along. Mention might be made about the occasional plant that simply didn’t
‘take’ to an aquarium, with no reason given for why it might not. The implication is that a species not
surviving is the exception.
I believe it is not exceptional, rather it should be expected.
Certainly different species require different amounts of light, tolerate different water chemistry, trace
minerals, to give just a few factors affecting growth. But even these factors don’t explain to me why
some plants flourish and others don’t.
My 16 gallon tank has about 10 species of plants which thrive, so I must be doing something right.
Nonetheless, a number of plants wither rather quickly in there. Among the ‘easy to grow’ plants that
die in there are anacharis, frill, and varieties of valisneria.
Something serious is missing from the literature and many who’ve had difficulty with ‘easy to grow’
plants might agree.
The plant merchants either don’t understand this because they grow plants in mono-culture
environments or they’re not telling you because they want you to keep buying plants. Also, it’s difficult
to determine which plants might co-exist because the environment depends on a great many variables,
including the mix of other plant species and types of nutrients you supply.
Competitive Advantage
I think you’ll have more success with your aquatic plants if you view them as competing species – a more
Darwinian view. They all compete for the nutrients you supply. The nutrient levels set by the aggregate
of plants will favor some species over others. Those species which like the nutrient levels thrive; those
needing higher levels of certain nutrients do less well.
In particular, for any collection of plants with a consistent nutrient supply, certain species have a
competitive advantage over others. If left unchecked they will dominate and those species with the
disadvantage will die out.
1
Incompatible Aquarium Plants
Plants with a competitive advantage harm other plants indirectly. They are more efficient with certain
supplied nutrients and set those nutrient levels lower than other species can tolerate. Those that need
higher levels of nutrients will diminish and die.
It’s not all fight or die, plants can coexist. You should identify which have the competitive advantage in
your tank and don’t let them get away with setting the nutrient levels too low.
As an example, suppose we have 3 tanks with similar water conditions, temperature and nutrient
supply. Tank A has 3 species and tank B has four species. These are established tanks whose species are
coexisting very nicely over a long time period. Tank C starts with all 7 species from the other two tanks.
What happens? It’s conceivable that all 7 thrive, but it’s far more likely that some do not. Perhaps only
4 or 5 will have competitive advantage; the others will slowly dies off.
My Attempted Experiments
I’ve tried a number of one-on-one experiments to see if I can find pairs of plants that don’t coexist, but
my attempts were all inconclusive. I blame that on poor experimental design. Nonetheless, my
experience with my regular aquariums gives me confidence in this theory.
What Mathematical Models Tell Us about Competitive Advantage
Mathematical modeling of competing species can get quite sophisticated but the conclusions are simply
stated. They illustrate the diversity of possible outcomes, more than enough to cover what occurs in my
aquaria.
The basic ideas are that plant species grow and die at different rates as they utilize nutrients. Plant
nutrients consist of plant fertilizers with a variety of compounds. Additives such as Flourish or Pro-Plant
add essential nutrients. Substrates such as Flourite add minerals. Carbon dioxide is also a nutrient. Fish
waste and unconsumed food are others. The models use several nutrients without being specific. That
aside, the models do present the consequences that are of interest to those of us trying to grow aquatic
plants. The models are well suited to aquaria since the nutrients are dissolved in the water or substrate
and are somewhat equally available to all plant species.
Dynamical System Models of Plant Competition
The rate of growth of each species depends on its ability to utilize the nutrients. Its decay rate is
proportional to the population size. The nutrients in the system increase with the supply rate and
decrease as the plants absorb them.
This is an example of a dynamical system. Starting from a set of initial conditions the system evolves in
time. In general the evolution of dynamical systems can be quite complicated, but for plant models they
all tend to very well-defined states, called steady states, as time runs on.
2
Incompatible Aquarium Plants
Steady states are those which don’t change with time. A simple example is when there’s just one plant
species and the nutrient supply balances the decay so the plant mass is sustained. Another would be if
two coexistent species had the right nutrient supply to balance their natural decay rates.
Two Plant Species Competing for One Essential Nutrient
I begin with a model that’s too simple to be practical, but worth study since it illustrates ideas central to
the more useful models. Consider two plant species A and B competing for a single nutrient supplied at
a constant rate. This might apply in a situation in which all nutrients can lumped together as one.
Suppose one starts with a mix of plant species A and B, what happens? Two steady states exist in this
simple model, one with all A and no B, the other with all B and no A. Wherever one starts out, the
system will evolve in the long run to the steady state whose species is more efficient.
Species A is more efficient with the nutrient than plant B if the ‘all A no B’ state has a lower nutrient
level than does the ‘all B no A’ state. If A eventually sets the nutrient level lower than species B
requires, B will die out if the system continues unchecked. I say that species A has a competitive
advantage over species B.
Coexistent Species
The simple model explains some things, but not when species do survive together. So a more
sophisticated model has two species, A and B, and two nutrients. What happens?
One possibility is that species A is more efficient with both nutrients than is B. This isn’t much different
than the Two Plant/One Nutrient model. B is at a severe disadvantage.
To make things interesting species A has to be more efficient with one nutrient, say NA, and species B is
more efficient with nutrient NB.
If one nutrient is supplied at a significantly greater rate than the other, the species favoring that nutrient
will have the competitive advantage. Again, it is like the first model except the supplied nutrient ratio
selects which species will survive.
But the new feature is that if the ratio of nutrients is in an intermediate range the species can coexist.
Between them they set the two nutrient levels and both survive.
Interestingly, the steady state nutrient levels are independent of the nutrient supply rates, determined
only by attributes of the plants. The supply rate determines how much of the plants you get.
Another interesting feature is that the coexistent nutrient levels are the same whether one has lots of
plant A and little plant B or the other way around. The supply of nutrient determines the mix of plants,
not the nutrient level.
So, increasing the nutrients you currently supply won’t alter the competitive advantage. It can only
increase the amount of plants.
3
Incompatible Aquarium Plants
In many cases the competitive advantage is independent of the quantity of plants. It depends only on
properties of the plants you have and the nutrient supply ratios. Having a lot of plants at a competitive
disadvantage means it will take longer for those with the advantage to set their nutrient rates, but
doesn’t affect which have the advantage or disadvantage.
Adding a Little Bit of a Third Species
The Two Species/Two Nutrient Model can be adapted to show what happens when a small amount of a
third plant species is introduced. If the new species requires nutrient levels greater than that set by the
coexisting ones, it will die out. If it tolerates the nutrient level, it will thrive.
I believe this explains what happens when I put a few plants of frill in my aquarium. The dominant
plants, such as wisteria and moneywort, set some nutrient levels too low for frill and it quickly dies.
More Sophisticated Models
One can analyze more sophisticated models with three or four species and two or three nutrients. In
each model efficient plants can coexist at nutrient levels which others can’t survive.
With more plants than nutrients, say a Three Species/Two Nutrients or a Four Species/Three Nutrient
Model, an interesting feature arises. This is worth noting because I presume there are fewer essential
nutrients in aquarium plant food than there are plant species.
With the simpler models, the All-of-One-No-Other steady states occur only when the ratio of nutrients is
skewed to one or the other nutrient. But with more plants than nutrients, one species can have the
competitive advantage with nutrient supply rates somewhat balanced. The dominant species is more
efficient with nutrients in the mid-range than are the others.
In my well-planted 16 gallon tank water wisteria (hygrophila difformis) certainly has competitive
advantage. I have to pull out healthy plants just to keep them from overtaking the tank. With too much
wisteria, I get fewer of other plants.
So What Do I Do When my New Plant Isn’t Faring Well?
So, what do these models suggest when you’ve put in a new plant and it’s not fairing so well? An easy
thing to do is to reduce the quantity of dominant species plants. By thinning out those plants that set
the nutrient level, you give the new species a better chance.
Increasing the amounts of the nutrients you supply won’t help. What might work is to change the types
of nutrients you add. If you know the new plant requires special minerals, add more of those.
But nothing’s free. If you switch to a new fertilizer regimen you may get your new species to thrive at
the expense of certain established plants. That’s why I haven’t fussed too much with the nutrients I
supply to my aquaria. I like the plants I have. If I change the added nutrients, I might make it unsuitable
for one of the plants I like.
4
Incompatible Aquarium Plants
Competitive advantage refers to the plants that will thrive in the ‘long run.’ But your aquarium need not
experience that ‘long run.’ By regular care you can keep a healthy mix of plants by constantly keeping
the system in a transition state.
As an example, consider my 16 gallon tank with 10 species of plants. The steady state it tends toward
might be the All-wisteria-nothing-else stable steady state. By regularly discarding Wisteria plants I keep
it well away from that state and all my plants (those that ‘take’) do quite nicely.
So, not much of this is specific, but you gain an understanding of what is happening when plants thrive
or not in your aquaria.
An Example
The following figure displays possible plant combinations in one Four Species/Three Nutrient Model.
The three triangle vertices correspond to the three nutrients. Species 1 is most efficient with nutrient 1,
and similarly Species 2 with nutrient 2 and Species 3 with nutrient 3. Species 4 isn’t more efficient with
any one nutrient.
To understand this figure, start in the lower left corner which represents a diet heavy on nutrient 1
favoring species 1. Moving to the right along the line represents adding nutrient 2. For small amounts
of nutrient 2, species 1 has the competitive advantage, but at some amount both species 1 and 2 share
that advantage over species 3 and 4.
From midway on the lower side, moving into the triangle interior represents adding nutrient 3 to the
mix. Again, small amounts don’t change which species have the advantage. But for some amount,
species 4 will be included among those having competitive advantage with the mix of nutrients.
Depending on the amounts, more of nutrient 3 will either restrict the competitive advantage to species
1 and 4 or to species 2 and 4. Arriving in the middle of the large triangle, with a ‘balanced’ mixture of
nutrients, species 4 is the most efficient and has the competitive advantage.
Increasing the percentage of nutrient 3 will move from where species 4 has the only competitive
advantage to where it shares with species 3. Increasing the percentage even more will make species 3
the only one with competitive advantage. Other mixes of the nutrients will give other sets of species
with competitive advantage.
5
Incompatible Aquarium Plants
3
3&4
4
1&2&4
1
2
1&2
All four species can coexist, but at least one is at a competitive disadvantage in regard to the others. So
in the ‘long run’ one species will die out unless steps are taken to prevent that.
Certain combinations with competitive advantage don’t occur. There is no 1 & 2 & 3. This is an
unstable steady state for this system. (More about stability in the next post.)
The 1 & 2 & 3 & 4 state doesn’t exist in the model. One would need at least four nutrients to get a
steady state with four species. I doubt that it would introduce any insights that aren’t contained in
these simpler models. Again, the goal is insight, not specific information.
As you can see, a variety of situations occur with just three nutrient components and four plant species.
In a real aquarium, one should expect no less complexity.
So, which plants do you think have a competitive advantage in your aquarium? Which have a
competitive disadvantage?
Next month I’ll present the mathematics of competing species for those who are interested.
Until then may your plants grow bountifully, especially those with a competitive disadvantage, and your
fish swim happily.
A Wisconsin Aquarium Hobbyist
6