UG filters work great. The entire bed of sand or gravel has tons of surface area for the bacteria to collect and begin their work. The black eye that UG filters have comes when the bed becomes "clogged" Meaning that the water stops passing down through the bed in spots or all together. The bacteria can't do their work and fish die. The bed becomes clogged because of all the debris and waste that naturally falls on top of the bed and over time tends to clog the water paths. To keep the UG working you need to do cleaning on a regular basis.....or else, fish die.
Ah, but that is where the reverse flow comes in. Yes, you are correct, instead of pulling the water down through the gravel bed, the water is pushed up through the bed. So why is this better? Well, the water that is pushed up through the bed can be filtered water. That is, rather than just using a bubbler or even a power head sitting down on the UG filter, you can feed the UG filter with the water that has been Mechanically filtered from an external canister, cartirdge or box filter. In this way, the water that is feed to the bottom of the bed, to be passed up through the bed, is "clean" of debris that would clog the bed. The other added benefit is now there is not the current of water pulling debris onto the top of the bed compacting it and making it difficult to clean. Instead there is an oh so gentle movement of water upward from the bed. This upward flow may help the mechical filter collect debris so that it can be removed.
I'm currently building a custom reverse flow UG filter for my 135 Gallon Salt tank.
Hope this helps
