I current have a 75 gallon, unplanted aquarium in which I have NO gravel at all. I removed the gravel years ago because I liked the clean look… and not having to vacuuming the gravel to remove debris. I really liked that look for awhile, but now I'm looking for a change.

I would like to cover the bottom of the aquarium with a thin layer of gravel, but I am trying to avoid the chore of having to vacuum it again. The gravel would be for aesthetic purposes only (I am not using it to anchor plants).

Would a water pump (or the output of my canister filter?) circulating water UPWARD through an underground filter likely negate the need to vacuum a very thin layer of gravel at all?

The gravel would only need to be a fraction of an inch thick (just enough to cover the undergravel plates). I am not planning on using it to anchor plants. Is this something people do? Does the equipment for this type of setup/configuration already exist, or am I going to have to jury rid something to give it a shot?

  • So you're looking to shoot anything in the gravel up. Then the idea is that it would be picked up by the canister filter's intake? Hopefully there's an answer that can address this, and I might try to when I have time, but my first thought was that most fish might not appreciate the updraft/current. But if you haven't selected fish yet you have the option of looking into getting fish that specifically prefer stream environments. Then it might not be a concern at all. – Spidercat Aug 26 '14 at 21:56
  • What if you had a mesh layer with room below it (like a crawl space or rat cage), put rocks that do not go through the mesh holes on top and a sucking filter hose under it? – Deirdra Strangio Aug 27 '14 at 19:58
  • @MattS. The updraft will be negligible; certainly WAY less than the output of the average spray wand of a canister filter. At my filter's max capacity of 350 gal/hr, that's about 1-1/2 cups per second spread over the 5.5 square feet of gravel (approx). It will be barely noticeable. – Robert Cartaino Aug 28 '14 at 20:09
  • @Raystafarian The water certainly wont be dirtier. My canister filter has kept the tank very clear for a very long time. Not all aquariums include live plants as part of the water cycle. Very interesting idea about the faux-gravel/sand bottom though. I will take a look – Robert Cartaino Aug 28 '14 at 20:14
  • @Raystafarian Bio filtering is well established: water chemistry always spot on; ammonia and nitrites always zero, infrequent partial water changes keeps nitrates at zero. I appreciate the note of concern, but to put those concerns into context, I have decades of experience raising and breeding fish and this is a long term, well-established healthy aquarium. I didn't intend to get so side tracked assuming problems that did not exist, but it's all good. – Robert Cartaino Aug 28 '14 at 23:12

It does provide some great ways to get rid of ammonia and nitrite. Also, the upwards flow of water through the gravel pushes solid waste into the water column so it's easier for the canister filter to remove the solid waste. You shouldn't get cloudy water or grubby gravel, assuming of course you clean the mechanical filter media. You can clean the canister filter as often as you want without worrying about killing bacteria, because there will always be lots of good bacteria in the gravel. Some Con's to reverse underwater filtering. Two of note. Firstly, they're essentially incompatible with plants, or at least plants with roots (they're fine with floating plants and epiphytes such as Java fern and Anubias).

Secondly, they only work properly if the gravel bed is more or less flat and open. You can't cover much more than, say, 15% of the substrate with rocks, and if the gravel bed is particularly thin anywhere, the water will mostly go through that area of least resistance, by-passing most of the filter bed. all of the are from: http://www.wetwebmedia.com/fwsubwebindex/fwugfiltfaqs.htm

It is possible to avoid the biggest problem of UGs, their excellent mechanical filtration potential, by reversing the flow. That is, instead of pulling the water down through the gravel, you push it up through the gravel. You also use a sponge pre-filter on the powerhead, so it is pushing pre-filtered water down the riser tube, out under the plate, and up through the gravel. This is reverse-flow undergravel (RFUG) filtration. Several brands of powerheads can be adapted to this, but I only have personal experience with one, Penguin, which offers an accessory kit to convert their conventional pump to reverse flow. The kit has an elbow attachment to mate the pump output tube to the riser tube and a sponge assembly for the intake side of the pump. This is not expensive, and being a lazy man, I haven’t tested any other technique since these came on the market. The sponges are easily removed for rinsing under the tap. There is no concern with preserving any nitrifying bacteria in residence in the sponge, as the gravel bed is to perform that service. The sponge can function purely as a mechanical filter, and be rinsed weekly, biweekly, or whatever your water change interval may be. I like biweekly at least, weekly is better, to get waste out of the tank before it is completely digested in the system. The gravel is still hydrovacuumed, but you will be astonished at how little material comes out of the gravel compared to conventional flow UG, or to conventional substrates.

From: http://badmanstropicalfish.com/articles/article64.html

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I experimented a lot with undergravel filter for the last months also for the purpose of keeping the gravel clean and avoiding maintenance with a vacuum cleaner. The effort was large, but result is imo astonishing: There's no visible dirt on the surface (I can even observe it disappear) and I can hardly get any dirt into a vacuum cleaner.

That leads me to say, that I don't see a reason why you'd want to inverse the sense of water flow. Furthermore the following arguments speak against it:

  • Water sinks if you pump the water out of the filter chamber under the gravel. That leads to a quite equaly rate of flow on the whole gravel surface (and thus the cleaning effect). If you pump the water in the chamber instead you'll loose that effect and only use a part of the gravel as filter media.
  • You need a very strong pump that either has to work against a huge pressure in you undergravel filter chamber or will lift your gravel and destroy the effect of a gravel filter.

Notice: I ended up creating a custom filter chamber with flexiglass with drilled holes adjusted to the gravel size - you can't take sand, but drills go down to 1mm which allows you to use 1mm gravel. Details at https://richtercloud.de:446/dokuwiki/doku.php?id=tasks:aquarium:undergravel_filter. In another aquarium I have a standard filter chamber (see https://www.amazon.de/gp/product/B009F8V3FY/ref=oh_aui_detailpage_o04_s00?ie=UTF8&psc=1 for details) which provides the same results, but doesn't allow small gravel and even the large gravel will cause blockage over long time. I use Eheim Universal pumps with 300 l/h which are expensive, but great.

Note that you need to remove all NO3 which isn't contained in organic material which you'd usually vacuum through water changes. Using a bucket for the cold water coming out of the shower at the morning dramatically reduces additional consumption.

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