How to keep plants healthy without growing algae

How to keep plants healthy without growing algae

  • Author ruud
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Disclaimer: this article is not intended to represent the accurate state of affairs in biology. This article simplifies in order to recommend a simple action for non-CO2 injected planted tanks, with a very high degree of success. The recommendation is to dim the lights.

This wisdom is mostly based on personal and second-hand experience, with CO2 injected and non-CO2 injected tanks, in very hard water to very soft water, densely planted to no plants, many fish to no fish, over-the-top fertilization to water changes only, algae infestation to algae free, and of course, bright lights to very dim lights. And many combinations of these variables.

Dimming lights​

Plant health versus plant growth

Plant growth and plant health have an opposing relationship to light intensity:
  • More intense light stimulates growth.
  • More intense light challenges health.
Plant growth is not a sign of plant health.

Low CO2 conditions

This simplification is particularly valid in non-CO2 injected / low CO2 concentration tanks. In low CO2, plants are less tolerant to light intensity.

Under normal, out-of-the-box aquarium lights (= too intense), plants can't help but grow. Because CO2 supply is low, this involuntary urge to grow causes physiological conflicts.

Typical signs of bad plant health include abscission (shedding of leaves), dark spots or patches and algae infestation.

CO2 injection

If CO2 is injected, plants are much more tolerant to light intensity. In both directions. You can increase the intensity without impairing health, but you can also decrease the intensity to a bare minimum. Obviously, you won’t see much plant growth by decreasing the intensity, but plants remain healthy.

CO2 injection and dim lights seems an odd combination, but in nature, this is nothing out of the ordinary. CO2 concentration of water is typically higher than in our tanks and sunlight is typically blocked due to the presence of trees and turbidity of water.

Algae do not compete with plants indirectly (for resources). There’s no such thing as a sweet spot where plants flourish and algae don’t. Algae require much less CO2 and nutrients and are much more responsive to changes. Algae and plants have similar responses to light spectral signatures. And both algae and plants are fine with low light intensity. Algae can actually prevail in very low light intensity. In sum, plants are no match for algae.

But all hope is not lost. The solution is to prevent algal spores from germinating.

Most algae species in our freshwater tanks exist as algal spores that are triggered by a combination of intense light and organic compounds or ammonia. The combination acts as a trigger: "spring is here, let's feast".

Dim lights, a bacteria rich environment, and a densely planted tank with healthy plants, lower the chance these spores germinate to algae. Note that dim lights affects the others in a positive way: dim lights > healthy plants > bacteria rich environment. Bacteria oxidise organic compounds and ammonia, thereby preventing algal spore germination.

Hence, dim lights have a double effect!
1. reduces the chance of algal spore germination, directly.
2. benefit plant health, which reduces the chance of algal spore germination.

Light intensity

Keeping the light intensity at about 10% of normal output is a good starting point. This should equal around 5-10 lumens per liter of water. From there on, you can gradually increase the intensity and observe the consequences.

That's it. Good luck!


Why are new tanks more prone to algae?
Newcomers typically purchase a few plants, that were likely cultivated emersed, and abruptly drown these plants in their aquarium under normal (=much too intense) aquarium lights. This is double terror.

Plant parts decay, which releases organic compounds and ammonia. The tank might not have a decent bacteria population yet, or worse, the owner is in the midst of chemical warfare for sake of cycling. Hence, plenty of ammonia for algal spores to transform to algae.

Are taller / normal fish tanks suitable for plants?
The distance between substrate and atmosphere and the distance between substrate and light source increases somewhat (owners respond by increasing the light intensity), which makes taller tanks a bit more challenging, but far from insurmountable.

What if I want a densely planted tank?
Either have more patience, because some growth does occur, or add more plants from the start or in time.

How about minerals?
Although 95% of plant health issues is related to CO2 and light intensity, people most often point the finger towards nutrients (minerals). However, plants don't require many nutrients in low CO2 conditions.

Water changes with perhaps some additional potassium is all you need. If you don’t keep many fish, you need to add nitrogen. Adding a bit of NPK liquid or KNO3 dry powder in the water is most likely all you need in non CO2 tanks. Along with water changes for all the other minerals.

How about the quality of lights?
Plants don't care much about spectral signatures. Plants are capable of compensating for signatures that deviate from the most efficient, thanks to auxiliary pigments. Not even full spectrum lights are required! Just about any light works for plants, as long as the total photon flux density is above the so-called light compensation point. In other words; it is only light intensity you should worry about.

It's hard to believe with all those dedicated and pricey plant lights available, isn't it?

How about yellow-ish or white-ish lights?
Choose according to your personal preference. Your plants don't care.

How about the duration of lights?
Plants and algae share a circadian rhythm. Simply comply with your natural day/night cycle. Why do some people deem duration to be important? Because it affects the light intensity. Turning off the lights, typically implies abrupt dimming, as the room continues to illuminate the tank.

Hence, in brightly lit rooms, you might not even need a dedicated aquarium light to keep and grow plants.

How about light interruptions?
Short interruptions, like siestas, might or might not affect plants (debatable), but it doesn't affect algae. Long interruptions, like blackouts, affect "adult" algae (gametophyte) a lot more than plants. Plants are capable of extracting energy out of produced sugars, allowing them to survive a blackout.

What if I like to increase the intensity?
The message in this article is simplified. Biology is an interacting myriad of gradients. General remarks:
  • Up to a certain point, algae can be kept in check with help of snails, shrimp and perhaps some fish. Start with very dim lights and gradually increase the lights and observe the effect.
  • In a densely planted tank, the canopy catches most of the light intensity. The canopy is closer to the water surface, hence has a little more access to CO2. Plants closer to the substrate catch less light and have less access to CO2. This can work.
  • Floating plants can also be used, albeit these plants have access to atmospheric CO2 concentrations, thus grow faster and use up a lot more nutrients.
  • Under intense light, organic compounds and ammonia can be kept in check by thoroughly keeping tanks free of waste. This is the normal procedure of high tech planted tank enthusiasts, who work with intense light.
  • As a low tech planted tank enthusiast, myself, I actually prefer the waste as it feeds microbe and detrivore populations. Hence, I focus solely on keeping the lights dim.
  • In both scenarios (waste free versus dim lights), a densely planted tank with healthy plants, both plants and oxygen-fed microbes, will absorb ammonia and convert organic waste to inorganic compounds.
  • It is very plausible for healthy plants to produce repellents that combat algae. Plenty of studies have shown this.

As a beginner, what is the best way to start?
  • Plants purchase are likely cultivated emersed. Hence, let them float in your tank or in separate vases, under dim light, for a few weeks. After a month, you can plant them submersed.
  • A densely planted tank works much better than a sparsely planted tank.
  • Because plants won't grow much under dim lights, purchase many plants.
  • Some plants are labelled as 'easy', others as 'difficult'. It is often said that difficult plants require better lights, a special soil, and CO2. The fact is, difficult plants, like Rotala Wallichii, simply cope less with low CO2. That's it. So choose 'easy' to 'medium' ones, if you don't intend to inject CO2.
  • Keep the lights dim, conduct your weekly water changes and add some extra potassium, and with minimal or no fish, also extra nitrogen. In case of doubt, use an all in one fertilizer.
  • Don't worry about dosing extra; fertilizers are inorganic compounds; it won't cause algae spores to germinate. They do feed existing algae. Understand the difference?
  • If plants start to show growth, you can gradually increase the light intensity. But you don't have to. It's mostly for aesthetics or to reach parts below the canopy.

What if I like to have red plants?
Light intensity brings out the red in a lot of plants. In low CO2 conditions, either have a densely planted tank that is better able to cope with light intensity and / or buy plants that are red-ish by default, like Ludwigia sp. mini super red. You cannot aim for aquascapes with strong red colors you find on the internet, without C2 injection. Repeat: CO2 injection allows for increased light intensity.

What if I like to have a carpet?
A carpet dominated tank is already a bit challenging in CO2 injected tanks with intense lights. I would not pursue this in tanks without CO2 injection. In a tall/normal fish tank, just forget about it. Typically, you need intense light to reach the substrate and keep the plants low. Hence, a lot of maintenance is required to not set off algal spores. At the very least, alter your expectations big time. You can have a bit of fun with species such as Monte carlo (be very patient) or those belonging to the genus Lilaeopsis.
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very useful, thank you!!!
Not only straight to the point, but also covers topics I wish would be more widely known ... or better said, the contrary, outdated "knowledge" would disappear already. Looking at you, light colors, and the ol' "Plants only use red and blue light spectrums"
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Excellent write up, well written, easily understood and implemented. Thank you for the time and effort!
For someone who is new to planted tanks, this is very straightforward and to the point. I have already appreciated your responses to my posts. This just adds to it all. Great job and thank you for your knowledge!
Got to say this is a very nice to the point article. Thank you!
This is a simple, straightforward, and informative article. Definitely worth reading. Thank you.
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