How your Pond or Aquarium Filter Works

When you are just starting out or even just thinking about taking up fish keeping for a hobby, it could be that your knowledge of filters can be summed up by a simple statement: Pond and aquarium filters are ‘magic boxes’ that clean my water.

There’s no shame in this, especially if you are only a budding aquarist, and most of us have been there at some point. We thought it would be a good idea to take a run-through of the basics of water filtration to allow you to grasp the fundamentals.

What Do Filters Do?

There is little difference between pond filters and aquarium filters in terms of their functionality – Both exist to clean your pond or tank water of unwanted substances and dirt. The primary need for adding a filter to your water systems is to remove ammonia from the water. Ammonia is produced by the organisms in your pond, primarily in fish excretions and from decaying plant-matter. It is poisonous to your fish, burning them due to its’ alkaline nature, inhibits their breathing (fish need oxygen too) and it acts as a fertiliser to algae, meaning that having high levels of ammonia usually results in large algae blooms and fish death.

In simplistic terms, filters clean the ammonia from the water as well as other unwanted substances and dirt, and this is done in two different ways.

Mechanical Filtration

Mechanical filtration is the simplest form of filtration. The Filter Media in your filter-box acts much like a kitchen sieve, allowing water and small particles to pass through and back into the main body of water, while retaining larger particles like algae clumps, dirt and debris. It is for this reason that you need to wash your filter media from time to time to stop the filter from clogging up, replacing the media itself when it gets too dirty.

It is important to wash your filter media only in used aquarium or pond water, and this is because you don’t want to damage the cultures of beneficial bacteria that live inside it that are a part of the biological filtration system, which we will talk about next:

Biological Filtration

This is where your pond or aquarium filter really becomes important. As we mentioned before, ammonia in your water is incredibly detrimental to your water quality, harming the life in your water (including your fish) and clouding the water with algae blooms.

Your filter media has a large surface area to volume ratio, with plenty of nooks and crannies (‘quiet areas’) for cultures of beneficial bacteria to develop, and in reality, it is these cultures of bacteria that are technically the ‘filter’ itself, breaking down the unwanted ammonia.

What this all comes down to is the nitrogen cycle of your water. Nitrifying bacteria in your filter media convert ammonia into Nitrites (technically worse than ammonia, but an essential part of the process) and then into Nitrates (pretty harmless in comparison). Nitrates can then be removed using water changes and some are absorbed by your plant life in your water.

Filter Media

Different filter media are adapted to different tasks between mechanical and biological filtration. Here is a brief run down:

  • Course & Fine Sponges: Provide a great level of mechanical filtration, with plenty of air pockets to develop multiple cultures of beneficial bacteria. A combination of both fine and course is usually the most effective
  • Ceramic/plastic media: These come in various shapes and sizes, usually in forms with a high surface-area-to-volume ratio to allow more cultures of nitrifying bacteria to accumulate. They help with mechanical filtration a little too.
  • Filter Carbon: These pieces of activated carbon absorb chemicals like dyes and pesticides as well as metals that can have a negative effect on water quality.

Multistage filters are usually the most effective, utilising all these different kinds of media to really get the water quality to a fantastic standard, and are even better when used in conjunction with a UV Clarifier.

What Do UV Clarifiers Do?

Often considered an additional part of the filtration system, but also often built-in to a many models of filter, UV clarifiers help with your algae problem.

Just keeping your ammonia levels down won’t always keep your both algae free. Some of the ammonia will never make it to the filter, and algae will develop and give your water a green-tinge. UV clarifiers work by letting the water flow past a Ultra-violet light emitting bulb that ‘clumps’ the free floating algae particles together so they can be captured by the filter media (mechanically) and then broken down by the bacteria cultures in your media (biological), giving your filter a much welcomed helping hand!

It’s best to do plenty of research while you are choosing the right pond filter or aquarium filtration system, and there is still lots to learn, but at least after reading this you might have a little more knowledge about what areas you need to look for.


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