Tropical Fish Forum

Tropical Fish Keeping => General Fishkeeping Chat => Topic started by: daveyng on October 29, 2018, 10:11:05 AM

Title: CO2 dispersal from an aquarium
Post by: daveyng on October 29, 2018, 10:11:05 AM
Probably a stupid question, but what happens to the CO2 that finds it’s way up to the surface of an aquarium. I believe that CO2 is heavier than air so technically a layer of the gas could form just above the surface of the water. It obviously doesn’t or the fish would suffocate. Presumably it’s dispersed by air currents etc.
Title: Re: CO2 dispersal from an aquarium
Post by: Sue on October 29, 2018, 10:31:40 AM
CO2 is slightly heavier than oxygen (about 1/5 air) and nitrogen (4/5 air) but not much.
N2 = 28
O2 = 32
CO2 = 44

The difference in weight in not enough to cause the CO2 to form a separate layer

Individual gas molecules have kinetic energy and are constantly in motion. It is called Brownian motion, and this will carry the CO2 away from the tank until the air is a homogeneous mixture of all the gases. (According to my husband who has studied physics  :) )

Air comprises
Nitrogen ~80%
oxygen ~20%
argon ~1%
the rest - carbon dioxide, krypton, neon, helium, xenon

All mixed up together.
Title: Re: CO2 dispersal from an aquarium
Post by: Helen on October 29, 2018, 04:52:22 PM
Gaseous dispersal is one of the scientific explanations for having a siesta period. The molecular weight slightly influences the speed at which a gas can disperse. And in water this is slower than in air (due to the relative weights of the solvent (ie water is heavier than an oxygen / nitrogen mix). Aquatic plants produce oxygen / co2 faster than these gases can naturally disperse in water. It is suggested that after about 4hrs, the layer of oxygen surrounding an aquatic plant's leaves is sufficient to (at least partially) block the plant's access to the CO2 in the water. So by turning the lights out for a couple of hours, the plants start to respire and use up the o2, replacing it with co2, which they then use when the lights come back on.

Obviously the water flow around the leaves makes a difference to the dispersal rate and therefore the optimum photo periods.

And each plant type has different requirements and production rates too.