That's why we have written an article on this seemingly lost filtration method. Can compete with new tech filters?
What are Under Gravel Filters
Under gravel filters are filters that are placed under the gravel as their name hints. They have a pipe sticking up acting as an outflow and the intake is a plate where water is driven in under the substrate. These filters do not usually have filter media as your substrate acts as the media! Think about it, since the plate is under your substrate the water is drawn through it and thus it creates colonies of beneficial bacteria in your substrate. Another benefit of this is that because of the constant flow from under your gravel, dirt and waste build up is significantly less.
Although this all sounds good there are surprisingly enough some bad sides to these filters. For example if a component breaks or the outflow disconnects you have to lift the plate out of the substrate which will ruin and dirty your whole tank. Also plates only come in certain sizes and can't fit all sized aquariums to mention the factor of having a pretty prominent outflow to try and cover up. Finally any of you looking at using sand or other fine substrates would not be able to use this filter. This is because these substrates would fall through the cracks in the plate and clog it.
How they work
Under Gravel Filters are driven by an air pump and air stone much the way a sponge filter is. To explain the air stone is placed down the pipe and the air driven out of it creates suction which pulls the water through the pipe and back into the aquarium. In this case the suction pulls water under and through the substrate then a plate before it returns into the main water body through the outflow.
How to make them or buy them?
Yes this is in fact a DIY friendly filter that you can build yourself. A few bits of PVC pipe and a drill can get you most of the way there. All that really makes it up is creating an open outflow pipe with connections to pipes that will be under the gravel. The pipes that are meant to be under the gravel will need to then have holes drilled in them. Be careful not to make these holes to big otherwise even gravel substrate could fall through and clog the system.
These filters are notorious for being some of the cheapest filters on the market due to them being considered obsolete. But due to this categorizing these filters are found in less and less places for sale. However after a couple of searches you will have a few options to choose from. Take into account most will be from old brands and may be as the business suggests, old.
Are these filters extinct?
No they certainly aren't, as a matter of fact due to the quiet market now is a perfect time to pick up a cheap but effective filter! Yes they have their flaws but all filters do, so if you are planning on having a gravel or large substrate in a normal sized aquarium these filters are perfect whether they are DIY or store bought.