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Playing around with under gravel filtration.


George H
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You might want to consider putting something like plastic needlepoint canvas over the top of the tubes and then the gravel on top of that. You want water flowing through the gravel (kind of like a fluidized bed sand filter just using bigger gravel) and without a plate of some sort you're apt to end up with water channeling down just where the tubes are with little to no flow where the tubes aren't. By creating a water filled space under the gravel there's somewhat less risk of channeling. Sealing off the upward facing holes in your tubes might encourage the tubes to pull more from the sides also. 

Getting that even flow of water across/through all of the substrate without there being channels is the big challenge with an undergravel filter. Most of the suction in your system will come from the area nearest the uplift tubes. It's the easiest water to move. Water will take the path of the least resistance. If the first three holes in your tubes can provide all of the water the uplift demands, the other holes farther down the tube will largely just sit there with little to no water movement. Instead of using the whole gravel bed as a biofilter you may end up just the gravel above the first inch of each tube.

I've always liked the idea of the reverse-flow undergravel filters. Use the output from a good canister filter to force water up from underneath the gravel. The drawbacks of a conventional forward-flow undergravel filter is channeling. Water takes the path of least resistance and aquarium water is filled with debris. That debris gets lodged in the gravel, slowing or blocking that initial channel completely and then a new channel develops and the debris starts to accumulate there. With little to no water flow through the now blocked channel the biofiltering in that area largely stops. In your design the channels will form nearest your uplift tubes and then as they plug, the channels will move down the tubes until all are blocked. A reverse-flow system using debris free water (use a canister filter to remove the debris) largely solves that problem. There is no debris accumulating in the gravel. Anything that tries to settle in the gravel should be floated off by the current rising up from under the gravel. It should be easier to maintain a consistent flow of water through the gravel. If you've seen a fluidized bed sand filter, you're essentially doing the same thing just without moving the gravel.

In the past I've seen marine tanks where an undergravel filter was siliconed onto the bottom of the tank and then a two to three inch layer of coral sand placed atop it and then a reverse-flow system used to quite literally make a fluidized sand bed inside the tank. The sand is constantly in motion and aerated in such a fashion and expands several inches in volume. It's a complicated system to get dialed in just right, but can give you enormous biofiltering volume. You just can't keep any bottom dwellers in such a tank and your filter that removes the solids is apt to suffer damage from pulling in the rogue sand crystals that get too close to the filter inlet. It is somewhat mesmerizing to watch such a sand bed in action though. Even then channeling is sometimes an issue. but it's easy to see if one part of the bed isn't moving. Fixing it can be more of a challenge but a quick stir through the non-moving section can often get it moving again.

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22 minutes ago, gardenman said:

You might want to consider putting something like plastic needlepoint canvas over the top of the tubes and then the gravel on top of that. You want water flowing through the gravel (kind of like a fluidized bed sand filter just using bigger gravel) and without a plate of some sort you're apt to end up with water channeling down just where the tubes are with little to no flow where the tubes aren't. By creating a water filled space under the gravel there's somewhat less risk of channeling. Sealing off the upward facing holes in your tubes might encourage the tubes to pull more from the sides also. 

Getting that even flow of water across/through all of the substrate without there being channels is the big challenge with an undergravel filter. Most of the suction in your system will come from the area nearest the uplift tubes. It's the easiest water to move. Water will take the path of the least resistance. If the first three holes in your tubes can provide all of the water the uplift demands, the other holes farther down the tube will largely just sit there with little to no water movement. Instead of using the whole gravel bed as a biofilter you may end up just the gravel above the first inch of each tube.

I've always liked the idea of the reverse-flow undergravel filters. Use the output from a good canister filter to force water up from underneath the gravel. The drawbacks of a conventional forward-flow undergravel filter is channeling. Water takes the path of least resistance and aquarium water is filled with debris. That debris gets lodged in the gravel, slowing or blocking that initial channel completely and then a new channel develops and the debris starts to accumulate there. With little to no water flow through the now blocked channel the biofiltering in that area largely stops. In your design the channels will form nearest your uplift tubes and then as they plug, the channels will move down the tubes until all are blocked. A reverse-flow system using debris free water (use a canister filter to remove the debris) largely solves that problem. There is no debris accumulating in the gravel. Anything that tries to settle in the gravel should be floated off by the current rising up from under the gravel. It should be easier to maintain a consistent flow of water through the gravel. If you've seen a fluidized bed sand filter, you're essentially doing the same thing just without moving the gravel.

In the past I've seen marine tanks where an undergravel filter was siliconed onto the bottom of the tank and then a two to three inch layer of coral sand placed atop it and then a reverse-flow system used to quite literally make a fluidized sand bed inside the tank. The sand is constantly in motion and aerated in such a fashion and expands several inches in volume. It's a complicated system to get dialed in just right, but can give you enormous biofiltering volume. You just can't keep any bottom dwellers in such a tank and your filter that removes the solids is apt to suffer damage from pulling in the rogue sand crystals that get too close to the filter inlet. It is somewhat mesmerizing to watch such a sand bed in action though. Even then channeling is sometimes an issue. but it's easy to see if one part of the bed isn't moving. Fixing it can be more of a challenge but a quick stir through the non-moving section can often get it moving again.

Probably not going to worry with it on a not even 4 gallons of water with light stocking. I'm sure it will be good enough for what I'm playing with. 

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