AQUATIC HORTICULTURE
BY KAREN RANDALL

Filtration and the Planted Tank

One of the first questions I am asked by experienced aquarists who are planning their first aquatic garden is “What type of filter should I buy?” Often, my response is “What do you have on hand? Most will do.” Filtration is actually a secondary consideration in the planted tank. The plants and substrate in the tank provide an enormous total surface area for the growth of beneficial bacteria.

In addition, in the typical planted tank with a pH in the vicinity of 7.0 or a little less, most of the ammonia produced in the tank will be in the form of ammonium. This is the preferred nitrogen source for aquatic plants — in a properly stocked, heavily planted tank, most, if not all, of the ammonium produced will be immediately used by the plants and never even enter the nitrogen cycle. This explains why nitrate readings in many well-balanced planted tanks are extremely low, sometimes to the point that nitrogen must be supplemented. It also explains, in part, why a tank that is densely planted when set up and allowed to settle in for a couple of weeks before fish are added experiences few if any of the problems usually associated with “cycling” a new tank.

Do you have questions, want to discuss the issues raised in this column, or read the comments of other aquarists and the answers from columnist Karen Randall? You can do so by going to Aquatic Interactive.
So, what do we, as aquatic gardeners, want from a filtration system? Well, I have already stated that biological filtration is taken care of largely within the tank, but extra biological capacity can’t hurt, and can, under certain circumstances, provide a critical margin of safety. The most common type of chemical filtration, granular activated carbon, is not usually necessary on a continuous basis in a planted tank and may reduce the level of trace elements. Still, there are times when chemical filtration of one type or another can be extremely useful in the aquatic gardener’s war against algae.

We are left, then, with mechanical filtration. Mechanical filtration is useful to remove suspended particles from the water that can settle on plant leaves and reduce the amount of light getting to the plants. It has no negative impact on the system, and the same equipment can usually also provide necessary water movement within the tank.

What to avoid

Air-driven filters, whether in the form of corner filters, undergravel filters or sponge filters, are usually not the top choice for a planted aquarium for the simple reason that the agitation of the bubbles breaking at the surface causes the loss of much needed CO2. This, of course, can be mitigated by the use of supplemental CO2. The advantages of these types of filters are that they are inexpensive and not prone to mechanical problems. Some people with low light, slow growth systems are able to provide adequate levels of CO2 for their plants in spite of the use of air-driven filters. These aquarists usually have minimal filtration for the sizes of tanks in question. They also often use peat or soil substrates that, by their very nature, add to the CO2 already present within the system.

Undergravel filters are not the evil villains in a planted tank that some claim, but they do have their limitations. The main function of an undergravel filter is biological. It must be maintained through regular, thorough gravel vacuuming or it will not be able to continue to perform in the manner originally intended. Because it is not possible to vacuum a heavily planted tank deep into the substrate, the undergravel filter plate will become obstructed, decreasing its usefulness as a biofilter. If air stones are used, they cause the same problem as other air-driven filtration systems — they drive off essential CO2. This problem can, however, be avoided by using small powerheads instead.

If the flow through the substrate cannot be slowed sufficiently either by the aquarist or by the plants themselves, an undergravel filter makes it difficult to use the substrate additives that are so beneficial to root-feeding plants. These filters also limit the size of substrate that can be used to a minimum of about 3 millimeters. Anything finer than that will fall through the slits in the plate. Both of these problems can be minimized by placing bonded filter pad material over the filter plates before adding the substrate. This, however, limits the usefulness of the undergravel filter as a biological filter, begging the question, “Why bother?”

On a positive note, an undergravel filter that has been in use for a number of years can build up quite a bed of nutrient-rich mulm that can be very useful to aquatic plants. Many people with successful planted tanks with an undergravel filter in place have reached this happy state. Under such conditions, however, the undergravel filter is probably doing precious little in terms of filtration!

As far as undergravel filters are concerned, my advice is this: If you currently have one installed and you are happy with the way your plants are growing, leave it alone — it won’t hurt anything. If you are setting up a new tank, however, particularly one intended as a planted tank, there is nothing that an undergravel filter can do for your tank that can’t be accomplished better and more easily by another form of filtration.

The fluidized bed filters becoming so popular for fish tanks have little benefit over other forms of filtration for our purposes. The quick transformation of ammonia (ammonium) to nitrate is not an advantage in a planted tank, and the main claim to fame of these filters is their tremendous biological capacity.

Multipurpose filters

Internal power filters can certainly be used in a planted tank, but must be removed for cleaning and are hard to hide in a display tank. They can, however, be very useful in special purpose tanks. In a paludarium, for instance, an internal power filter that can be placed on its side is a big advantage in a shallow water area. They can also make sense in a grow-out tank, where looks aren’t the main concern.

External power filters are probably the best choice for the smaller planted tank. The best ones include some permanent form of biological filtration, such as a sponge or biowheel. Some types have replacement “cartridges” that contain carbon (they can be rather pricey), so you may not want one in the planted tank. Others allow you to replace just the media pads in a plastic frame. Try to find one of the latter types.

A good external power filter should have a space for chemical filtration media for the times when this is desired. At other times this space can be filled with coarse gravel, a sponge or “bio-beads” for use as bacterial starter cultures for other tanks. External power filters are relatively easy to clean and do not impinge on space within the aquarium the way internal filters do. Because most are designed to “spill” water back into the tank, they do have a tendency to produce surface turbulence that drives off CO2. This can be mitigated to a large extent by keeping the water level in the tank as high as is practical, so water slides back in gently.

Canister filters are probably the filter of choice for most aquatic gardeners, particularly for larger tanks. Styles vary, but the best are quiet, relatively inexpensive, have good media capacity and are easy to service. Quick disconnect fittings are well worth the small additional expense and make filter cleaning even easier. Make sure the return is positioned well below the water surface to minimize surface turbulence.

For the largest tanks or for those who want to push the limits on fish stocking, trickle filters may be the best choice. Commercial units can be expensive, but plans are available that make it possible for a reasonably handy person to build his or her own. These filters can strip a lot of the CO2 out of the water unless the trickle chamber is air tight, so CO2 supplementation is almost a must in the return line.

One of the advantages of this type of filtration is that heaters, CO2 reactors and so on can be mounted in the sump, removing even more equipment from the tank. Another feature that some people appreciate is that water changes and topping off chores can be done via the sump without ever disturbing the main tank.

Mechanical filtration

Many hobbyists think of sponge filters as primarily biological filtration. While it is true that once colonized they provide a large amount of surface area for bacteria, in a planted tank they are better thought of as a form of mechanical filtration. They can be used to prevent powerheads from becoming clogged or can be placed over the intake of an outside power filter or canister filter for the same purpose. Prefilters of this sort can be particularly useful if you are battling algae. If you use sponge prefilters and rinse them at least every other day, you will remove a lot of material from the system before it can be broken down by bacteria to become algae food!

Diatom filters can be useful in planted tanks (as in other tanks) to “polish” the water after maintenance, to remove algae blooms and to get rid of any laterite that you inadvertently get into the water column! People who use soil substrates find that if they run a diatom filter for a few hours after disturbing the substrate (during planting or whatever) they can forestall the onset of an algae bloom.

Chemical filtration

In my opinion, most chemical filtration media are better used to pre-treat tap water than to solve problems in the aquarium itself. Regular partial water changes (25 to 30 percent weekly) should be enough to prevent the buildup of unwanted substances within the tank. Still, there are times that chemical filtration can be useful even on the tank itself.

Granular activated carbon may remove needed chelated trace elements from the water, something we certainly want to avoid. Therefore, it is not recommended for use on a continuous basis in a planted tank. Those who use either peat-based substrates or quantities of driftwood may find that their water turns yellowish or brownish. Carbon in the filter for even a few hours can usually solve this problem quickly. It should then be removed. Carbon can be used in the same manner to remove certain medications from the water if (heaven forbid!) you need to medicate your planted tank for any reason.

Some people will find that they have high enough copper levels in their tap water that it is troublesome for some species of plants and even some fish. These people may find that a copper-removing resin, such as “Cuprisorb,” used in a holding container before the water is added to the tank is very helpful.

Nitrate- and phosphate-removing resins can be useful either for pre-treatment or for use directly on a planted tank with a larger than optimum fish load. If your tap water contains unacceptable levels of either of these substances, prefiltering the water is a good option. Don’t be fooled into blaming all phosphate and nitrate problems on your tap water, though. If the levels in your tank are higher than the levels in your tap water, at least some of your problems are coming from other sources. It is still easier and cheaper to reduce these levels with water changes directly from the tap.

One type of planted tank that can often benefit from the use of phosphate- and nitrate-removing resins is the planted discus tank. Discus are large fish that eat meaty foods rich in phosphate. Unless it is a really large tank with just a few discus, it is likely that you will be battling to keep your phosphate and nitrate levels down on an ongoing basis even with frequent, large water changes. In this case, you may find that the use of these resins can help you keep ahead of the game in between water changes. Just remember that the goal is to keep nitrate and phosphate levels low enough to avoid algae problems. It is not necessary to totally remove every last bit of either substance from the water.

No filter at all?

There is also the very real option of no filtration. A number of aquarists have found that a healthy planted tank that is lightly stocked functions very nicely without any filtration other than that provided naturally by healthy growing plants. In this case, a powerhead or two can be beneficial to provide water movement.

Innovative filtration options

Finally, some innovative aquarists are working with algal turf scrubbers and terrestrial plant filters. Both of these options have some strong potential benefits for fish tanks or those planted tanks with a higher than optimal fish load (like the discus tank mentioned above). It must be remembered that the plants in these filters, whether they are algae or higher plants, will also remove CO2 and other nutrients from the system, so their use on the average planted tank is probably counter productive.

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