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Plants and Filtration

What is the best type of filtration for planted tanks?

By Karen Randall

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Q. I am planning to set up a planted aquarium, which has led me to some questions. I'm a bit puzzled about filtration in aquariums in which aquatic plants are the focus. What is the purpose of a filter in such a setup (compared to a community aquarium for fish)?

What types of filtration would be best? Are there any kinds to avoid? Would a wet-dry or fluidized bed filter be okay? I've observed that wet-dry filters drive off carbon dioxide and raise the pH. Do fluidized bed filters do the same?

If I choose not to use a filter, would this impact what I might place in the aquarium? How about using lava rock or feather rock in such a setup? Would they add any phosphates to the water?

A. There are several reasons to use filtration for a planted aquarium. First, and probably most importantly, it provides water movement. Aquatic plants are able to take up nutrients much more efficiently in at least gently flowing water. Water movement also prevents suspended particles from landing on aquatic plant leaves where they can interfere with photosynthesis. Even if the flow is set low, it will keep particulates from settling on the substrate, so they can remain in circulation and be removed by the filter. Also, the water flow from your filtration system can power a carbon dioxide (CO2) supplementation system.

As we know, in addition to water movement, filters can provide mechanical, chemical and biological filtration. Which are of importance in a planted aquarium? Most people who keep fish in their aquariums want some mechanical filtration. Leftover fish food, fish wastes and dead plant materials all contribute to detritus in the planted aquarium. This can, of course, be handled with regular siphoning of the aquarium, but most people prefer a filter to help with the job.

Chemical filtration is rarely necessary in a well-managed planted aquarium, particularly after the initial set-up period. Still, it is nice if your filter has a chamber that can be used for chemical filtration should the need arise.

Biological filtration is the primary reason for filtration on a "tropical fish" aquarium. It is nitrifying bacterial filtration that makes a freshwater fish aquarium habitable in most cases. In a planted aquarium, things are different. Aquatic plants are "biological" filtration too! If the plants are growing well and the aquarium is properly stocked, plants are actually a better form of biological filtration than bacteria.

While bacteria are very good at converting ammonia (actually, ammonium) to nitrite and then to nitrate, there the process stops. They do nothing to reduce nitrate or phosphate levels in the planted aquarium, nor do they remove any other potentially harmful substances (like excess metals) from the water.

There is also an enormous amount of surface area for nitrifying bacteria to colonize in a planted aquarium, and the bacteria are competing for ammonium that the plants need for good growth. For these reasons, additional bacterial filtration is not a high priority.

While there are many different filters that can work adequately in a planted aquarium, my choice for aquariums between about 50 to 100 gallons is canister filters. For smaller aquariums I prefer appropriate outside power filters, preferably the types with adjustable flow rates that do not require specially made disposable filter media "cartridges." For very large aquariums, wet/dry filters are a viable option.

Fluidized bed filters are not a particularly good option for a planted aquarium. The purpose of these filters is almost entirely in the realm of bacterial filtration, an area that we want to minimize in a planted aquarium.

Wet/dry filters can drive off carbon dioxide unless they are tightly closed to hold carbon dioxide in. The pH increase is due to this loss of carbon dioxide. Most people running planted aquariums large enough to need wet/dry filtration will probably also find that their aquariums do better with a supplemental carbon dioxide system. This system should be in the return line from the filter to the aquarium to maintain the carbon dioxide levels and pH desired. While there are other reasons to avoid the use of fluidized bed filters in a planted aquarium, they are unlikely to drive off carbon dioxide.

It is certainly possible to run a successful planted aquarium without any filtration as long as the aquarium is stocked conservatively. For reasons already stated, however, water movement is beneficial. I would run at least a powerhead (more than one in a large aquarium). A powerhead fitted with a sponge filter is a good alternative in a planted aquarium. For maximum benefit, the sponge should be rinsed every few days so that it is working more in a mechanical capacity than as biological filtration.

You'd have to check your rock source to see if it will leach phosphate. But, with all the surface area available on the plants themselves, there is really no need for any extra area for the colonization of bacteria. Let the aquatic plants do their job!

Filters to be avoided in the planted aquarium include any that cause surface turbulence, which can drive off needed carbon dioxide. This includes air-driven filters, such as bubble-up corner filters, air-driven undergravel filters and filters that return water via a spray bar above the water surface.

If an undergravel filter is used, it is best run with slow-flow powerheads set low enough in the planted aquarium that they do not cause unnecessary surface turbulence. Undergravel filters have the added disadvantage that they limit substrate amendments to those that will not be kicked up into the water by the water flow through the substrate.

In spite of what I have said here about the "best" forms of filtration for a planted aquarium, please be aware that for every type of filter, there is probably someone, some place who is using it on a beautiful, healthy planted aquarium. If you own a filter that is not one of the "best" for a planted aquarium, there are almost always ways to either modify the filter or modify your maintenance routines to make the system work.

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