Undergravel Filtration has been used for many years as the principal means of biological filtration in the marine aquarium and despite the development of newer rival systems it remains the most popular option today. There are two methods available, the first being called ‘The Downflow Method‘.

The Downflow Method

Downflow is the traditional and most widely adopted method due to it’s understandable set up. Here is a step by step guide of how to get started when using this method.

Step 1 

Start by covering the base of the tank as completely as possible with undergravel filter plates. These can either of the small, lock together variety, or the larger all-in-one with fine slits.
Tip: Some aquarists prefer to glue these to the base using silicone sealant.

Step 2

For a four foot tank, an uplift should be fitted to the plate(s) at either end in the back corner.
Tip: The tubes may have to be cut with a hacksaw to accommodate a powerhead on each one.

Step 3

Once the plates are in position, cover them with a suitably coarse medium such as a coral gravel, crushed shells or dolomite chips at a rate of 10lb (4kg) for every square foot of base area; 40lb (16kg) in total.
Tip: A gravel tidy, which is no more than a fine plastic mesh, must be then placed over the coarse media and neatly trimmed to fit tightly into all the corner and around the uplifts.

Step 4

Cover all the gravel tidy with a layer of coral sand, again at a rate of 10lb (4kg) per square feet. The mesh will prevent the two media from mixing together to form a compacted, ineffectual filter bed.
Tip:  It is wise to wash all media thoroughly prior to positioning and to give it a thorough visual examination for foreign objects and contaminants.

Step 5

The power heads should draw the total water volume of the tank through the filter bed at least three times every hour. If several power heads are used, they must all be of the same make and power rating to provide maximum efficiency. The coral sand will require raking through a regular basis to prevent compaction through on a regular basis to prevent compaction, which reduces the through-flow.
Tip: Any mulm and detritus can be siphoned off the same time.
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