It has been awhile since I have been involved in any forums like this so I am a bit rusty and my previous response was not as clear as I should of made it. So let me be a bit more concise on what I was thinking and sharing. As to the comment about bacterial colonies I was making a point about northern climates and the fact that regardless if the bacteria colonies are dormant or dead, they simply aren't functional or working optimally for a signficant part of the year. Therefore if you are looking to capture organic mulm as the byproduct of heterotrophic bacteria, it seems problematic to expect this to happen except during the time periods where the bacteria is actually fully operational. As I mentioned, this is a concern more driven by where I live and work and thus not as much of a concern for where the original poster lives. On the subject of whether or not this type of setup would work or not (and not if it should be installed or not), I would suggest the answer is tied to finding proof that heterotrophic bacteria will colonize and thrive in the environment provided in this type of design. I have serious misgivings based on the research I have done on this subject. Yet I have no proof for my thoughts. That alone is a serious problem and underscores the challenges we face in the waterscaping market. What are my misgivings? Based on research papers and technical articles I have read, a gravel substrate of any kind does not provide enough surface area for bacteria to colonize in significant numbers as compared to more advanced media types (bio balls, ceramic media, beads, ribbon, etc...). In addition, the health of any colonized media is dependent on the amount of nutrient and oxygenated water moved over and through the media. So regardless of what type of bottom drain present (gravity fed or pump driven), the movement of water through the entire "system" of gravel would be essentially a column directly above the drain. With no method of moving water laterally through the entire gravel bed, you are counting on water circulation in the pond to take care of that chore for you. Yet water acts in a laminar manner and unless you have some external force thoroughly mixing all the water in the pond and then drawing it down over the entire surface of the gravel bed, there would be areas of stagnation or lower optimal performance. The comment was made about the koi keeping the debis stirred up and the bottom and sides being sloped to encourage movement into the drains but this confuses me. If you are installing essentially a false bottom in the pond, the koi will never be anywhere near the bottom or the drains and thus would have no influence on moving debris into the "cone of influence" necessary for the drain to capture. Furthermore, the slope seems inconsequential to me since the gravel bed is acting as a settling chamber that blocks the movement of debris down to the drain. If as stated previously the goal is just to capture mulm, you still need a mechanical means of pushing the mulm through the gravel bed, especially once you have a significant amount of physical debris on the surface of the bed (such as during the fall/winter when no actual bio conversion is taking place in my theoretical northern pond). In my understanding of the physics of fluid dynamics (which may be flawed), you really need to force the water through the gravel bed in much the same way a plunger pushes water through a pipe or opening. In the case of a plunger, air is the driving force to move the water. Yet the only force acting to move mulm downward into the sump area under the gravel bed is gravity but there still needs to be some inertial force present to break the bonds that cause the mulm to cling to the gravel. Again I reference the comment about the action of koi to move the mulm or turn it like a compost pile. I won't argue that koi could stir up and even enhance the bio process on the surface of the gravel bed. But koi can't stir up and turn over the debris that moves between the gravel and sits in the interstitial areas. Unless you can install the gravel in a perfect layer only one stone thick, you are creating the potential for debris to collect, decompose and cause problems if it cannot be converted fast enough. If you can install the gravel in a perfect layer, then I think back to my initial thought regarding this idea and that is if a gravel bed can provide enough surface area for the bacteria colonies to function optimally. The other comment that I failed to clarify is about just that...clarity. The water in a pond can be crystal clear and still it can appear "dark". The bottom of a natural pond or lake can be visible to depths of 20 or 30 feet. I don't dispute that. But you also have a huge amount of surface area to allow light in. My thought on this subject was that in a typical pond, the bottom and sides will be covered in fine hair algae and even a bit of sediment (biofilm). Therefore when it comes to the aesthetic concerns of seeing the bottom/liner/gravel, it doesn't take much time before a gravel bed or a smooth liner look the same once the depth approaches 3' or greater, no matter how clear the water is. So that leads my mind back to the question of whether there are other more optimal ways to provide supplemental filtration without the possible negative consequences that may result from an in pond gravel filter. Finally, the statement was made about such a system not being needed but rather would it really work or not? Again, this statement is the most problematic of all to me because none of us can really answer it. You ask for documentation to be posted in proof of a yes or no answer. As a pond installer, I have been hoping for such proof for most every aspect of our industry for as long as I have been involved in it. Where is the proof that any filter works, that any pump moves the amount of water it says it does, that beneficial bacteria really speeds up the bio seeding process, that UV lights "kill" harmful organisms, that a pond has too many koi, or that one type of media works better than another? The only proof we have is the experience we have gained from our own activities and the information we share with each other. Not one single product I use in my jobs comes with any documentation or proof that it works. What our industry lacks is concrete research backing up the claims of products, manufacturers and installation methods. So what we have here is our own research lab where we discuss ideas and theories. If we really want to find out if this idea works or not, then we have to build it and compare it to other ponds. My response and involvement in this thread are influenced by the understanding that I simply don't have the resources to test out every theory I want to and so in this case I am applying the knowledge I have gained from other applications and forming nothing more than an opinion that this setup would not perform well enough to justify the cost and/or time (installation plus maintenance) required to include it in a pond. That is all the proof I have (either positive or negative) and that will gnaw at me until we as an industry decide to do something about setting standards and creating a real and tangible way of "proving" what does and doesn't work.