Why plants grow better with CO2

Perhaps you remember our post Waarom is kooldioxide (CO2) de belangrijkste plantenvoedingsstof in het aquarium? , which was preceded by the series Het sprookje van de vloeibare CO2-bemesting ?

In the course of our tests we were able to gather some measuring data which we don't want to keep from you.

The discussion, whether aquariums also work without CO2, is as old as modern aquatics itself. One thing is clear: aquatic plants need carbon for their growth. Of course, aquariums also function without CO2 but in comparison the plants look weaker and duller and they don’t grow as vigorously. Several plants cannot be cultivated at all without CO2. This limits the choice of plants. A foreground lawn or a red eye-catcher in the aquarium would be unthinkable without CO2.

The easiest way to make it clear is with a comparison of the biomass. For that reason we set up three aquariums and determined the initial weight of the plants, which was identical in all aquariums (species/weight).

Aquarium 1: blank test without CO2, lighting 48 W (2 x 24 W JBL SOLAR TROPIC ULTRA fluorescent tubes) on aquarium with 63 l gross volume.

Aquarium 2: with a Bio CO2 system, lighting 48 W (2 x 24 W JBL SOLAR TROPIC ULTRA fluorescent tubes) on aquarium with 63 l gross volume.

Aquarium 3: with a compressed gas CO² plantenverzorging system, lighting 48 W (2 x 24 W JBL SOLAR TROPIC ULTRA fluorescent tubes) on aquarium with 63 l gross volume.

The initial weight of the test plants had a drained net weight of 300 g (which means moist but without dropping water anymore). After 4 weeks the aquariums were emptied, the plants (incl. roots) carefully removed from the bottom material and the algae from the plant mass, as far as this was possible. The drained net weight was determined, as before the setting up. The aquarium was fertilised with JBL Ferropol to ensure the trace element supply.

Blank test without CO2 supply JBL BioCO2 JBL compressed gas system
Initial weight plants [g] 300 300 300
Final weight biomass in total [g] 320 440 840
Final weight algae [g] 76 7.1
Final weight plants [g] 244 440 832.9

The blank test (average CO2 content in water 6-8 mg/l CO2) showed only a small increase of biomass of 20 g in total. The actual plant biomass decreased by 9 % (56 g), whereas the algae biomass increased to 76 g. Even if you can’t transfer the results to all aquariums (especially for those with weaker lighting) in general, this shows clearly that it resulted in a shift of plant-based biomass (reduction of taller plants, significant increase in algae). The leaf distance of the stem plants, the leaf colour and the stem structure varied greatly to the plants in the CO2 supplied aquariums and had a rather weak and dull impression. If a plant aquarium is left to function without CO2, there will only be very simple, low-growing plants left and there will be a great risk of algae formation.

The aquarium with bio-CO2 supply reached an average CO2 content of 20-24 mg/l CO2 over the test period. Here you can discern an evident increase of 46% from 300 g to 440 g (equivalent to 140 g increase) and no visible increase in algae.

In the aquarium where the CO2 supply was adjusted to about 38-40 mg/l CO2 through a compressed gas system (controlled by the JBL ProFlora pH-Control Touch ) you could discern more than a doubling of the biomass by 177 % (approx. 533 g) in the taller aquarium plants. Only a low amount of 7.1 g algae could be removed from this aquarium at the end of the test, and this was of no statistical or optical importance because of the strong increase of the plant biomass.

Granted, the third aquarium with the compressed gas system was producing large amounts of CO2, because the average recommendations lie between 20 and 25 mg/l CO2 (or extended between 15 and 35 mg/l). Therefore in practice, as controlled during the auto pH mode on the JBL ProFlora pH-Control Touch , CO2 contents of 20-25 mg/l are set, which are sufficient to ensure very good plant growth at average lighting intensity.

Based on the results you can see that the differences are marked. Even the aquarium without CO2 has changed- but with quite a lot more algae mass and very low plant growth. The leaf distance of the stem plants, the leaf colour and the stem structure varied greatly to the plants in the CO2 supplied aquariums and had a rather weak and dull impression. If a plant aquarium is left to function without CO2, there will only be very simple, low-growing plants left and there will be a great risk of algae formation.

© 31.08.2016
Ralf Rombach
Ralf Rombach
Dr. rer. nat.
JBL GmbH & Co. KG

Forschung & Entwicklung, Wasserpflege, Biozide, Düngung, Aquaristik, Gartenteich

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