Can I run my filter in reverse flow?

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I love experimenting with filtration and wonder if I could connect an external filter to the uplift of an undergravel filter for improved filtration?

You can and this sort of thing has been done before. At the end of the undergravel filter boom in the 1980s and early 1990s many were looking to boost performance.

An external filter can be connected to an undergravel with a simple reducer aimed at pond piping. An uplift pipe is a standard 25mm diameter and external hoses tend to be 12 or 16mm. Get a 25-16mm or 25-12mm reducer and join them with flexible PVC tubing.

Use hose clips and/or silicone for extra fixing. Failing that, Eheim used to produce a reverse flow undergravel kit. From memory, however, it was very expensive.

Before setting up, decide whether you want to run the undergravel in standard or reverse flow. The latter is best in terms of filtering as the external acts as the mechanical filter, trapping the waste for easy removal and then pumping the clean water through the gravel bed where the biological filtering will occur.

With water pumped through the gravel, detritus should be kept in suspension and removed by the external. However, with external return pipe connected to the undergravel, you lose surface agitation, so combine it with a small pump or air stone for increased gaseous exchange.

Using the filter in standard mode with the undergravel acting as the pre filter isn’t so good as the waste is trapped in the gravel and needs to be removed.

The filtering capacity of a combined undergravel and external filter becomes massive - so why aren’t we doing more conversions?

Advances in aquatic plant culture suggested that undergravels were bad because of the flow of water past the plant’s roots – though I have seen plants growing well in reverse flow situations. For marines, undergravels and biological filters were discarded for much more natural, live rock filtering.

If wanting to use reverse flow undergravel for a non-planted, tropical or coldwater situation go for it, but expect people who haven’t kept fish for 20 years to be confused when you explain what you’re doing!

This item was first published in the August 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.