Bradshaws Banner

Order Bradshaws Catalog button


Algae & ponds

Ponds & filters

Pond plants

Ponds & water gardens

Pond & fountain pumps

Top ponding tips


Pond liner sizes

Tensor pondvac

Oase Pondovac 3

Hozelock Cascade pumps

Oase pressurised filters

Maximus fountain pumps

Olympus Filter Pumps


Ammonia Pollution Toxic Chemicals & How To Avoid Them In Your Fish Pond

All fish ponds are self polluting. The first pollutant to occur in a pond containing fish is a chemical called ammonia. This is made up of mainly nitrogen. Fish excrete ammonia through their gills as part of the natural metabolic process. After eating the level of ammonia excretion increases. The problem with ammonia in a pond is that it is toxic in quite low concentrations and gets more toxic as the pH of pond water increases.

This article delves a little more deeply into ammonia, what it is in a pond environment and under what circumstances it is really dangerous. By following this short article you will be well able to identify and handle potentially dangerous conditions developing in your pond. You will appreciate the importance of a biofilter and how you can control and modify biofiltration efficiency in your own garden fish pond.

In the early days of a pond and its filter and if you have added lots of fish persistent and high ammonia concentrations can only be handled in one way: by changing large portions of pond water on an ongoing basis.

In an established pond with filter problems ammonia will also be evident and will remain and continue to build up until the cause and cure of the problem has been identified and rectified. If this situation is ever reached in your pond then STOP feeding the fish and start changing water quickly. Quite simply the ammonia comes from the food we feed our fish.

  • All fish and humans for that matter need proteins in order to grow big and strong. All proteins contain amino acids which all contain nitrogen which becomes the source of the ammonia we get in all fishponds. The trouble with ammonia is that it is poisonous to fish. This sounds crazy - a fish eats food to grow but in doing so sows the seeds for its own death by poisoning.

  • Of course this is not what happens in nature because the levels of ammonia do not build up to poisonous levels. This is achieved in a lake for example by restricting food supplies, limiting fish population and ensuring there is a biological balance that prevents ammonia building up.

  • In a pond however ammonia build-up can be dramatic and deadly. To understand what can happen and to therefore prevent dire consequences a basic understanding of ammonia chemistry in water helps.

  • If ammonia is present in water (this means every pond with fish in it) it can be present in 2 forms ... ammonia which is deadly or ammonium which tends to be a far less harmful

Ammonium is continuously transforming itself into ammonia and hydrogen ions in the water and vice versa. In any situation this transformation reaches an equilibrium or balanced state. Changing the situation by changing pH or temperature for example will disturb the old equilibrium and create a new one. Unfortunately some equilibrium conditions are far worse than others as we can see below.

  1. At a pH of 7 or less only ammonium is present (remember ammonium is far less poisonous than ammonia but still poisonous). Normal tap water should be around pH = 7.

  2. At a pH of about 8 around 5% is ammonia and 95% ammonium

  3. At a pH of 9 then 50% is ammonia and 50% is ammonium. At this point we are heading for big trouble.

  4. At a pH of about 11 there is no ammonium only ammonia. Your fish are also all dead.

The sad point is that there does not have to be a great deal of total ammonia to create havoc in a pond and the higher the temperature of the water (summertime) the less there needs to be even when the pH is constant.

At this stage I would like to point out that pH levels of 9 are not uncommon in ponds when there is a large amount of algae present and the water lacks what is called alkalinity.

Suffice to say large algae blooms are potentially dangerous to fish because they can cause sudden and severe increases in pH. This is another reason for installing an UV light even in the best koi ponds with excellent biofiltration.

Gerry Preston who writes superb articles on koi keeping in the UK magazine Nishikigoi International calculated that between 3 and 4% of the dry food fed to koi becomes ammonia that is secreted by the koi into the pond water. Gerry also showed that immediately after feeding the fish ammonia levels tend to be at their maximum. In addition he also showed that it is better to feed little and often rather than a lot at once in view of this spiking of ammonia levels.

All however is not doom and gloom (just that it can be and this is what I have wanted to show in this section).

The solution to this potentially disastrous situation of ammonia poisoning is the installation of a well-designed biofilter big enough to cope with the existing fish density and also the future stock density that will arise from the fish growing.

To finish off the section it is worth pointing out that the nitrogen contained in the ammonia actually sticks around in the pond water and is quite difficult to get rid of.

An installed biofilter however creates a situation whereby the ammonia is transformed first into nitrites and then into nitrates. This series of reactions is referred to as the nitrification cycle. Nitrification can only take place in the presence of specific bacteria accompanied by large amounts of oxygen. More about this later.


Best in Class

Pond additives

Pond vacuums

Water features & ornaments

Pond Filters

Fish food

Pond lighting

Pond plants

Garden ponds

Pond pumps

Pond Tools


Polyex liner exclusive to Bradshaws

Complete Pond Kits

Pond Treatments

Oase Pondovac 3


Site Map | Bradshaws store items