Diffusion Osmosis in a garden pond

In this series on Total Pond Management that explores a number of processes that interact to produce an aquatic ecosystem (and give a koi pond its characteristics), we will meet many processes and phenomena that operate largely unnoticed. All of these are processes are vital for maintaining the stability of life in a koi pond (if that were not the case, such processes would not exist) and arguably the most fundamental of the behind-the-scenes processes is diffusion, with its close relative osmosis.

Diffusion is an essential process in all environments, both aquatic and terrestrial, where it describes the mixing and movement of molecules (and microscopic particles) in a fluid.

The molecules of a fluid (gas or liquid) are constantly moving. The molecules are so tiny that we cannot see them moving, but we can often see evidence of their movement, and thereby observe diffusion in action. For example, if we turned off all pumps and aeration in a pond, allowing the water to slow completely and added a solution of malachite green in one corner of a pond, it would ‘move’ or disperse through the whole pond until it became a uniform solution. In natural clay ponds in Niigata, where a little mixing will occur through air and fish movement, the dispersion of minerals, odours and particles through a pond is due largely to diffusion. It is by diffusion that the appetising odour of food or the allure of pheromones will travel across large distances in a still clay pond. But more importantly, diffusion plays a key role in the lives of koi and other aquatic organisms whose job it is to process pond water.

Diffusion is a ‘passive’ process which means that it does not require any input of energy from living organisms such as koi or filter bacteria and will occur whether life exists of not. The temperature of a fluid (ie pond water) determines at what speed the molecules move in their random directions with faster movement (and rates of diffusion) occurring in warmer water. Imagine the balls in a bingo machine, colliding with each other in a random way. That is exactly what is going on in a fluid when viewed at a microscopic level – molecules moving in a random way and for diffusion to take place, there needs to be at least two different solutions that are free to mix (such as malachite green and pond water).

The random movement of molecules within these two solutions means that malachite green molecules will move into the pond water and pond water molecules will travel into the malachite green, blurring the edge between the two solutions until eventually, they become one weaker solution of malachite green. In this way, malachite green molecules move from an area of high concentration to an area of low concentration until the solution becomes uniform, a situation called equilibrium. This in-built tendency for solutions to ‘want’ to travel from an area of high concentration to low concentration through random movement is a process that keeps all organisms in a koi pond alive, including koi.

Diffusion and the koi pond.

Aeration. For water that is high in dissolved oxygen (such as the surface layers of a clay pond) and is adjacent to less well oxygenated water, the oxygen will move through the water by diffusion, distributing the oxygen throughout the pond. If this did not occur, koi (and other aerobic aquatic organisms) would only be found living in the surface layers of a pond.

Koi breathing. Without diffusion, koi would suffocate, even if the surrounding pond water was saturated with oxygen. Koi use diffusion every time they absorb oxygen from the surrounding water by ensuring that the oxygen concentration of the blood that passes through the gills (and adjacent to the pond water) is lower than that of the pond water. By doing so, koi create an area of high and low oxygen concentration, just the conditions that are required for rapid diffusion to occur. And because the koi’s circulation ensures that deoxygenated blood is always passing through the gills, there is always a concentration gradient between the pond water and gills, causing oxygen to diffuse into the blood.

The excretion of waste products. The reverse is true carbon dioxide and ammonia. Koi use diffusion to their benefit by passing blood through the gills that is high in waste products such as carbon dioxide and ammonia. Because the blood flows adjacent to the pond water that is hopefully low in carbon dioxide and ammonia, these toxic compounds diffuse readily from the blood into the pond water, freeing koi of these toxic compounds.

However, if due to overfeeding, overstocking or an immature filter, ammonia levels are allowed to rise within a pond, then ammonia will diffuse more slowly from the koi’s blood into the pond, causing the blood to recirculate back into the koi’s body high in ammonia, causing the koi further stress. Consequently, to allow koi to utilise diffusion for their benefit, every effort should be made to ensure that pond ammonia levels are kept at zero.

Other pond organisms. More primitive aquatic organisms such as bacteria, that do not have organs adapted for breathing or eating use diffusion as their means for gaining food and oxygen. Aerobic bacteria , being oxygen consumers, will generally have a low oxygen concentration relative to the pond water, resulting in a net movement of oxygen into their tissue. The same is true for other solutes such as ammonia and nitrite where there is a higher concentration in the water than the bacteria, thus ensuring a constant flow of desirable products into bacteria.

Diffusion and plants. It is by diffusion that blanketweed (with its very fine structure) takes up nutrients from the surround pond water, fuelling its growth. But thankfully, it is also by diffusion that blanketweed takes up herbicidal chemicals added to the pond water, resulting in its death and control!


Osmosis can be regarded as a close relative to diffusion, as it obeys similar rules such as random movement of molecules, moving from an area of high concentration to low concentration. The subtle differences are that osmosis only refers to the passive movement of water (rather than the solutes dissolved in it), as it passes through a semi-permeable barrier (or membrane) such as the gills.

Osmosis presents koi with a major headache, as compared to the pond water, their tissue has a low concentration of water, giving water the potential to move into koi from the pond through any semi-permeable membrane. The gills, unfortunately for koi, represent such a membrane. Gill tissue is so thin that it is only permeable to water and a limited number of small solutes, preventing many other salts and sugars from passing through. Consequently, koi attract water through osmosis and have to work hard to maintain a stable body tissue concentration. They achieve this by excreting large volumes of dilute urine, a process that costs koi considerable energy.

Some koi keepers opt to add salt to their pond water, maintaining a permanent yet very low salinity. The salt acts as a mild antiseptic and reduces parasite infections, but it also reduces the concentration gradient between pond water and koi tissue concentrations, reducing the rate at which water will pass across the gill via osmosis.

Yet it can be argued that koi are fully adapted to freshwater and should not require such supportive measures on the grounds of osmosis.

Osmosis and ulcers.

Ulcers allow unprecedented levels of water to enter koi tissues, something that can be reduced by adding salt to pond water as a short term measure until the ulcers heal. When adding salt to the pond to treat koi, we need to appreciate what effects the changes in salinity may have on the pond’s ecosystem as a whole. It will affect the rate at which other salts and minerals will diffuse into and out of other organisms as well as affecting their rates of osmosis (largely for the better). But this should be a reminder that under the philosophy of ‘Total Pond Management’ any action we take for the benefit of our koi is likely to have knock on effects (both positive and negative) for the host of other aquatic organisms that help maintain our pond’s stability. We need to act with care.

Osmosis and smaller aquatic organisms.

Osmosis poses a burden for most freshwater aquatic organisms, as all such organisms have a lower water concentration than the pond water. However, we can use osmosis to our benefit, by harnessing its power when treating against certain external parasites.

If we give the pond environment a high salt concentration than that of a parasite, water will move through osmosis from within a parasite into the saline water, causing the parasite considerable stress, and causing it to drop off its host. In extreme cases, the parasite may even implode. However, bathing a koi in a concentrated salt solution can also cause the koi severe stress and great care must be taken to observe the koi at all times during concentrated salt bath treatments.

Diffusion is a relentless phenomenon, acting on all solutions where there is a variation in concentration. It can play a vital role within a water body, working to homogenise a solution, and also forming the basis by which most living organisms interact with their environment.

Likewise, osmosis is a passive ’24/7′ process that affects all aquatics organisms from bacteria right up to koi. Most organisms in a koi pond have to expend energy to excrete the influx of water, thereby maintaining a stable body concentration. It can however, also be used to our benefit if a salt bath is used to treat against external parasites.

Kill blanketweed and string algae.