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  • Filters - Basic Guide

    Author: Adrienne Dodge

    Choosing a filter

    There are many varieties and brands of filter available to the fish keeper.  The type of filter most suitable for you and your tank will depend a lot on the tank size and the type of fish you keep.

    It is recommended that when purchasing a filter you choose one that will turn the volume of water in your tank over  4 – 6 times per hour ie if you have a 40 litre tank, choose a filter that has a flow rate between 160 – 600 litres per hour.

    There are references below to Biological, Mechanical and Chemical filtration.  This is covered under Cycling your tank.

    Under gravel or Undergravel filter

    These filters are the lowest maintenance and are generally the cheapest to keep running however they do not always work well in planted tanks as unless you have deep enough substrate the roots can eventually block the plastic grates.

    An under gravel filter is a filter where water is drawn through the gravel  - the gravel becomes your filter media.  When you purchase an under gravel filter you are purchasing a plastic grate (filter plate) which keeps the gravel off the bottom of the tank, lift tubes to deliver the water and a small length of plastic airline. 

    NB – you will also need to purchase, as a separate item, an air pump, some more airline and a check valve.

    The under gravel filter works by the air pump drawing water through the gravel (which acts as a mechanical and biological filter), up through the vertical pipes (called airlifts) and into the tank.  As the water is moved out of the lift tube it is replaced with water from under the filter plate – this water has been pulled through the gravel (mechanical filtration) which has caught any free-floating  waste particles.  The gravel, filter plate, tank bottom and lift tubes all provide a bed for bacteria (biological filtration).

    With an under gravel filter you do not have to change your filter media but you will need to clean the gravel regularly.  Using a gravel vacuum clean approximately 1/4  to 1/3 of the gravel every time you do your weekly water change.  If you do not the debris is likely to collect in the holes in your plastic grate. These will slowly block and eventually stop the filter from working.  



    Internal filters

    Internal power filters have a small water pump on top of a narrow box containing a sponge and sometimes other filter media.  These sit within the aquarium.  They are normally placed in a rear corner of the tank with the outlet just below the surface.  Some tanks come with fitted internal filters.

    Water is sucked in at the bottom of the filter and is drawn through a coarse sponge, trapping debris.  The sponges contain what is often referred to as good bacteria which converts nasties such as ammonia to nitrite and nitrite to nitrate (see Cycling your Tank).

    Every 4 weeks, or when you notice the flow of water being pushed out of your filter reducing, the sponge needs to be rinsed in old tank water to remove the debris (syphon out some tank water and rise the sponge in this).  If your sponge becomes blocked the filter will not remove as much waste and a slow running filter may also reduce the amount of oxygen in your tank.  Do not rinse your sponge under the tap as good bacteria will be killed off by the chloride in your tap water.

    Internal filters are fine for basic use but until recently most do not have much room for media inside them.  There are models now available with room for more media than a basic sponge.  They are cheap and easy to maintain on lightly stocked tanks



    External filters

    An external filter sits outside the tank, normally in a cabinet below.  Water is sucked out the tank and into the filter, where it passes through the media, before being returned to the tank..

    External filters have much more media inside them than internal filters, so they tend not to need cleaning quite as often. They are also much more versatile, and there is a lot of specialist media you can add to them to improve the quality of your water.

    If purchasing an external filter consider one with a self-priming mechanism; a device which draws water from the tank into the filer so it can start to pump the water back in to the tank.  This makes starting your canister filter easier.


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