From The Aquarium Wiki
What is it?
Bioload is a term used to generalise the amount of life existing in an aquarium.
Every animal in your tank produces waste in the form of solid or liquid excretions. This waste consumes oxygen as micro-organisms eat it and break it down into its basic nitrogen components such as ammonia. This ammonia is very toxic to most forms of life and needs to be removed otherwise it would quickly seriously harm or kill fish, shrimps, frogs, etc..
Every piece of food you add to the tank which is eaten by your animals, contributes to this waste as does any piece which is not eaten but lies in a forgotten corner of the tank and this becomes food for bacteria and fungus.
As the water in the tank can only hold so much oxygen, anything that consumes it other than the animals on display or the bacteria in the filter consuming the toxins, is therefore undesirable.
This amount of waste being made is really the 'bioload'. In a tank the most obvious signs of a bioload observed and measured is thre ammonia, nitrite and nitrate levels. These are combated by providing an adequate nitrifying filter to remove ammonia and nitrite. Followed by water changes to remove nitrate.
However, the bioload is never constant, as your animals grow you need to add more food to feed them and so they will produce more waste. If your filter is not working well (perhaps due to blockage) or is undersized to cope with the free toxins, this often allows algae and other excess bacteria to grow outside the filter therefore consuming more oxygen. So the bioload of your tank increases over time.
Owners who allow uneaten food or animal excretions to accumulate are also unwittingly increasing the bioload. An over population of snails in a tank due to uneaten food lying on the substrate will directly lead to more bioload and therefore more resources from the owner to keep it under control.
An experienced aquarist knows to keep a tank underpopulated, sparsely feeding to reduce waste, regular water changes and ensuring that the filtering system is more than equal to the amount of bioload they wish to keep.
Recognising high bioload
The first sign of a high bioload is that the level of oxygen in the water is too low to adequately support active animals in the tank. The animals keep near to the surface (if they can) and in the small hours of the night, the animals may be scrambling for air at the surface. But the owner all too often never sees this regular night time warning.
The second sign may be algae blooms due to free ammonia in the water and tuffs of brown bacteria appear on surfaces throughout the tank as the bacteria population climbs to take advantage of the free food.
There are methods of combating bioload. The most obvious is that you can remove some of the animals from the tank or you can keep overfeeding to an absolute minimum and perhaps introduce a fasting day once a week?
You can increase the size of the filter or place more efficient filter media in the filter so it can hold more nitrifying bacteria?
Artificially increasing the amount of oxygen able to get into the water by increasing the surface area by aeration or water movement is an obvious avenue taken by a great many novices. But this is a path that requires caution. Because if there is a power failure or some thing that stops these devices from working, then oxygen levels drop swiftly and the animals may suffer.
Performing regular water changes removes dissolved organic waste and increase the redox levels of the water so it can hold more oxygen is a simple and cheap method to combat excess bioload. But this routine needs to be done constantly and often beginners don't do it often enough.