Correct biological filtration is very important because it :

breaks down ammonia to nitrite (Nitrosomonas bacteria)
breaks down nitrite to nitrate (Nitrobacter bacteria)
oxygenates the water for the benefit of all lifeforms and the filter itself.
removes a number of unwanted compounds through facultative aerobic-anaerobic activity in portions of the filter (Wolff 1984).

Incidentally, and as a reminder that trickle filters are not some recent development, the first reference to such filters that I could find is in a book published in 1935, yes that long ago, called American Sewerage Practice, by Leonard Metcalf and Harrisson Eddy, Volume 3 of 3, The Maple Press Company, York, Penna. Call them if you are interested in how large volumes of water get treated. That book was brought to my attention by Matthew Cammarata.

The filters in that book are large, some even very large, at least compared to what we use in the Hobby nowadays, but the principle behind their efficiency has not changed an iota over the years.

Such biological filters (not necessarily all trickle filters) can be set up in many ways. All work of course, or Hobbyists would not have used them for several years now, and they would not be around in the literature anymore.

There are however varying degrees of efficiency in the types, brands and models available on the market. Some just do a better job than others. You know most of them :

Canister filters, traditional
Canister filters, special
Undergravel filters, regular flow, reverse flow
Outside box filters
Trickle filters in many configurations
and a combination of one or more of the above.

There are other types of biological filters too, perhaps not as familiar to you, but they are used by a fair number of Hobbyists too:

BioMesh filters
Ruecksack or Backpack filters (hang-on trickle filters)
Tray Filters, similar to trickle filters
Outside the tank biological sand filters
and a variety of intermediate ones, based on the sand filter system.

All of them serve the same purpose : the three reasons stated above. As indicated, however, some of these filters do a much better job than others at improving water quality, especially where it comes to re-oxygenating the tank's water.

For years in the hobby, especially in Europe, the trickle filter has made great strides in this country too. In 1985, when I first imported trickle filters from Germany (from a company called Nollman Aquaristik, in Sennestadt), hardly anybody even knew what such filters looked like, or what they were supposed to do. Nowadays, just looking at the ads in the Hobby magazines, everyone seems to claim "they" were the first to ever offer this type of filter (refer too to my earlier remark about the 1935 reference to trickle filters in _American Sewerage Practice_ to convince yourself that such is merely advertising copy).

Who was the first, and when, is not important and germane, what counts is that these filters are extremely efficient, and that for the purpose we are interested in - high quality water - they are the foremost filter, and the most effective at the present time. The variety and number of such filters now available attests to their success, and to the fact that both manufacturers, and Hobbyists alike, believe in them.

Because I strongly believe that to run a successful Reef you need such a filter, or a similar type e.g. a hang-on unit (Ruecksack or some types of conversion units, for instance), we will review only such filters in this section. Information on undergravel, and similar filters, can be found in many other books, as I am sure you are aware of. Refer to them if you still want more details, or if you want to review and compare them with trickle filtration.

Inside an appropriate compartment, a medium is placed on which Nitrosomonas and Nitrobacter can grow in large numbers. The shape of the box has absolutely no bearing whatsoever on the efficiency of the filter, although most manufacturers make them square or rectangular, because round tube or pipe of the size usually required, would make the filter much more expensive.

When dealing with the shape of a trickle filter, the important factors to take into account are the following, in my opinion:

filters that are too narrow tend to create "channeling" of the water against the outside panels of the filter, resulting in very inefficient use of the medium,

filters that are short, meaning not tall enough, do not promote re-oxygenation very well, and are therefore not as desirable. The minimum suggested height of the column of biological medium is 12 to 14 inches.

Most filters you see are clear, probably because clear acrylic is less expensive than other varieties. In fact, the filter would do better if it was not clear, as the bacteria perform better in the dark (documented in too many articles to mention). Nollman Aquaristik, of Sennestadt Germany, which has been building filters for many more years than anyone in the USA, builds theirs in opaque PVC sheet. Luebbecke Aquarium who have used large trickle filters on their 10.000+ gallon system for many years, uses 3 foot diameter grey PVC pipeline sections.

Another reason may be that clear filters can easily be looked into, something the Hobbyist likes to do, to determine how the water is flowing through the medium.

The biological filter compartment should be removable from the other parts of the filter. This allows for easier transport and shipping, easier installation, and much easier cleaning of the other parts of the filter when the need arises (and it will, believe me, mainly because most filters on the market do not have slanted bottoms which will require you to syphon out detritus from time to time).

As the water comes down from the tank to the filter, and disperses inside the biological chamber, it travels over the medium and allows the bacteria that grow on them to polish the water by first converting ammonia to nitrite (one type of bacterium) and then from nitrite to nitrate (another bacterium).

The better the dispersion of the water, the better the water is spread over the medium, and the better the filter will be able to clean up the water. That should be obvious. Not only is filtration more efficient, but by using the medium more efficiently you will need less of it. This can save you quite a few dollars as most of the media are still relatively expensive.

Additionally, less medium fits in a smaller filter, and that too will save you money. It will also make more space available underneath the aquarium, and since space there is at a premium....

Because you do not want to have to clean the biological filter - in doing so you would destroy a large amount of bacteria - the medium should allow a good throughflow of water, and not trap dirt that may have slipped through your fine filters (mechanical filtration). It isn't supposed to, but some will. If it gets stuck in the medium, in the long run you are looking at cleaning it. We already stated that such is not desirable at all.

Dense material will plug up no matter how well you filter. The main reasons for this is that fine particulate matter not trapped in the fine filters will eventually clog certain areas of your dense biological filter, creating anoxic pockets. When oxygen levels are low, or non-existent, decay is proceeding anaerobically, and hydrogen sulfide is produced. This will pull your water quality down very quickly.

In fact, if you were measuring redox potential levels, and they are low, even though you are using a considerable amount of ozone, the likely reason is that small amounts of hydrogen sulfide are pulling the redox down, because of the reductive power of hydrogen sulfide. Increasing the ozone may not even help. You must eliminate the H2S to bring the redox potential back up. There exist of course many other reasons for low redox potential, some of them will be covered elsewhere in this book, especially in the Maintenance Section.

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