I have just read this entire thread from start to finish- and let me just say; I respect how much thought has been put into this subject.

I have just read this entire thread from start to finish- and let me just say; I respect how much thought has been put into this subject. I started my own koi pond about 1 1/2 years ago and wondered why the filtering systems were so complex. I was hoping that there really is a simpler, yet still effective way to maintain water quality. It seems that the anoxic system is the perfect answer. Since size really does matter, I am already expanding my pond and am switching from adding a veggie filter to the planted anoxic filter.

It also seems that just one project is never enough, I just bought (before I read this thread) a 90 gallon aquarium that I will have mostly planted with a school of tetras. The "guy" at Big Al's sold me 3 boxes of laterite and 9 bags "Activ-Flora planted aquarium substrate" plus a whole box of various nutrient additives. It is all very complicated with a daily dosing regimen, yada, yada, yada. So of course I am now thinking of switching the tank to anoxic system. Any thought on how I would do that? The fish guy wanted me to mix the 3 boxes of laterite with 3 bags of substrate, and then put the rest of the substrate on top. Should I just mix the laterite with the kitty litter and put the substrate on top? Or is there some other way of doing this? I also have a wet/dry filter for the tank. I am picking up the plants for the tank Friday, so I need to make a decision soon. Any thoughts would be appreciated.

Thanks,
Katie

http://www.nfkc.info/nfkconliine/Cat%20Litter.htm

Katie,

Though this thread is not really about fish aquariums as-per-say, you do have a good point in asking if the Anoxic filter can be used in aquariums; which it has and still is being used in such. A 90 gallon fish tank with plants is not as easy as you may think. Without CO2 and a controller being added your plants will never grow to their full potential.  Over the long run your tank if not done right it will eventually collapse and it will have to be torn down periodically.

The odds are your fish-guy makes it sound like you add your substrate to the bottom of the aquarium with some Laterite underneath and the plants will do the rest at keeping the media oxygenated and clean. Through testing I have proven that this kind of setup does not work, and if it does it will be very short lived.

You see plants will draw water through the substrate through conventional means but the plants will be very slow in doing this and are subject to CO2 fertilization. Depending on plants alone to draw water through the substrate is a path to failure without the help like heating cables like is suggested by the experts. But even heating cables under the substrate becomes useless in the summer months when the ambient air temperature of the room equals that of the tank or the tank is getting enough heat from the aquarium lights. Then the heating cables never turn on to heat the substrate and then everything starts going anaerobic in the media and starts turning black.

Glass bottom aquariums will show this best and you can then see the blackening of the substrate very easy by looking at the bottom of the tank. Once the heating cables resume the work of heating the tank once again and your tank will turn sweet once again, then slowly the substrate will turn aerobic and you will have more aerobe organisms or facultative anaerobes than anaerobic organisms.  

The applied physics are the same no matter if it is an aquarium or pond. Plants alone will not draw enough water through a substrate to keep it from going anaerobic without a little help from the hobbyist. The substrate in a planted aquarium needs to be at least 3” deep and a plenum needs to be added under the substrate of ½ to 1” depth.  By doing this, heating cables are not needed and biological and chemical pathways remain open to chemical mediators. Now your substrate will draw water by convention, diffusion and peculation.

However, this will once again be subject to the size of the media you choose to use to anchor your plants in. If the medium is too small or too large then you’re sacrificing permeability and porewater capability of that substrate and it will clog. Unlike the Biocenosis Clarification Baskets (BCB’s) that make up the Anoxic filter in a pond, aquariums can’t have a bunch of BCB’s sitting inside of them because of cosmetic reasons. That’s why when you buy plants you will see them in individual small plant pots that act like a BCB’s does but on a smaller scale. They are grown this way in greenhouses with space in-between them just like an AFS has space between its BCB’s.

I have also seen hobbyist use cat litter and Laterite for their substrate in aquariums that houses plants too. But once again you must elevate the medium above the bottom glass surface like I explained above. Now, you will have an Anoxic Filter without all the BCB’s but will have one big BCB the size of your aquarium bottom instead. Using a black craft canvas on top of a platform will hold the cat litter and Laterite above the glass and because of its size it will not clog.

How to make a BCB the size of an Aquarium is easy. You can use PVC or anything that can make a raised bed above the aquarium floor. Then in one corner or in an inconspicuous place in the aquarium place a 1” plastic tube with small holes drilled at the bottom ½” of the tube. The tube is inserted in the raised bed and will sit on the bottom glass (The tube can also be capped off at the bottom.) with an air stone at the bottom of the tube and the tube will then look like and under gravel airlift tube. Using the smallest air pump you can buy, pump air into the tube via air stone. This will move water from under your plenum extremely slow. It will not act like an under gravel filter because water movement will be too slow for that to happen. The slower the water movement the better it works. Just the opposite of what an under gravel filter was meant for. If the tube only exchanges one gallon of water a day that’s good enough and fast enough too. Just make certain that none of the cat litter can fall in-between the tube and the black craft canvas screen.

The tube is added; because the substrate is tight against the aquarium glass at its sides. This will impede the chemical and biological pathways some. The very slow water movement underneath the plenum will allow for proper ion exchange and will mimic a conventional movement of the water through the substrate releasing any gases. Yes the substrate will produce N2 and it must find a way out via the tube you just added.