There have been more under gravel filters sold than any other. It's inexpensive, effective and out of site. However, no other filter today has sparked more controversy. Some hobbyist say the under gravels are outdated technology. Both sides of this debate have good points but I say that under gravels can still be used if one maintains them properly. So what are they and how do they work. As the name implies the business end of this filter sits below your tanks substrate. This filter uses your substrate as a medium for nitrifying bacteria just like a sponge filter. These filters can be air driven or driven by a power head (submersible pump).
This filter really requires water changes with a gravel siphon since all debris are drawn into your tanks substrate. This being the filters biggest fault. However do to new power head technology we can now use under gravels in a new way. Some power heads can be reversed. With a simple sponge pre filter under gravels can now be reversed. Why use reverse flow? As I said before the filters biggest fault was all debris getting drawn into your tanks substrate. With reverse flow all debris are kept at the top of your tanks substrate. This helps keep the debris in the top layer of gravel making water changes much more effective. In closing I would like to say that under gravel filters are effective if maintained properly, even though they have been around for years.
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