We will be discussing two popular forms of Filtration. The first is the Wet Dry system, while the second is the Fluidized Bed. This is not to compare different brands of filtration though. It is to show you some of the arguments as to why these two filters are so incredible.

The Wet Dry filter, also known as the Trickle Filter, has been very popular in recent years with aquarists ranging from Discus keepers to Saltwater keepers. Like most filters, the idea is simple. Bacteria need Oxygen in order to go about their activities, such as reproducing and converting wastes to different forms. The more Oxygen they get, the more efficiently they will do this, up to a specific point where they reach maximum efficiency, in which case the idea is to increase their numbers. The Wet Dry filter lets water pass through air (collecting more Oxygen) before becoming available for the bacteria, which are also exposed to some air as well. This gives the bacteria more energy, and leads to more efficient filtration. It also helps aerate the aquarium.

The Wet Dry provides remarkable Biological filtration, but it has some disadvantages. First, they need to be housed in a Sump, which means more piping, more space, extra pumps and wires, and extra expense. Since they normally sit below the tank, one has to find a way to hide the sump from view. Sumps do have some advantages though. They can be used to grow algae (on a reverse lighting cycle) to stabilize the pH and absorb wastes, a Protein Skimmer and Heater fit nicely down there, they give more water in the system for wastes to be diluted in, some extra space to put a bag of Carbon or Phophate-Zorb, along with other benefits. So this is not really a major drawback.

The maintenance to a Wet Dry is generally keeping the sump filled, keeping the pumps clean of debris, and cleaning part of the media out (such as the sponge in some) in tank water. If not cleaned, the Wet Dry becomes a “Nitrate sink,” to use the classic term, leaching Nitrates into the water. They collect detritus, which leads to massive algae blooms. Another reason for cleaning the media is that the bacteria can clump together over time at top, leading to the lower bacterial populations being less efficient. However, generally they are rather low maintenance filters. One caution is that if one is gone on a trip for a few weeks at a time, the sump needs to keep being filled while they’re gone. This may pose a problem for some people.

If the power goes off, then the large sponge (which is wrapped in a spiral and houses the main population of bacteria) can lose the bacteria population. Keeping the sponge wet during power outages is very important, or you will lose filtration for a few weeks while the population reestablishes. This may be a problem in certain areas or for certain people. Some Wet Drys do use at least some Bioballs though, which stay submerged and maintain their bacterial populations.

If you do go with Wet Dry, try to find the time to make it yourself. Many sites have a DIY section for them. If not, costs are very high, and 200$ (US Currency) for a nice system is not uncommon. The Wet Dry is an excellent filter, but recently they’re becoming replaced by most aquarists with the following filter:

The Fluidized Bed filter is relatively new to the mainstream hobby. They have many advantages over Wet Dry. The bacteria bed is always in motion, instead of being still like in a Wet Dry. This means all areas are utilized, unlike a Wet Dry where the whole media can never be colonized to maximum efficiency.

They are much cheaper by comparison. 75$ would be an average price for an average tank. They are generally around 95% smaller than Wet Drys, and are able to hold much more media per square inch. In fact, there are over 5,000 square feet of surface area for each cubic foot of media it is said! Fluidized Beds come in sump-form, canister form, and hang on tank models, so they are more versatile. Fluidized Beds are self-cleaning as well, and there are no centralized areas of bacterial build up. The Filters are the best artificial filters on the market, and are far more efficient than Wet Dry.

One disadvantage is that when the power goes out, the bacteria are covered in sand and water, and Oxygen depletes over time. Power outages are a problem with most filters though. Another disadvantage to them is that you may have to mail order them, because many fish stores may only sell one brand, if any, and do not let you comparatively shop. This may add some to cost. As they are becoming more popular though, this should be less of a problem.

The other main disadvantage is that the sand bed can get clogged. This is simply taken care of by placing a prefilter before the water goes into the sand bed. This is usually just a simple sponge, but must be cleaned and replaced frequently so that you maintain the maximum flow rate of water through the sand bed. Also, the idea is for the bacteria to colonize in the filter, and not on that small sponge of course. New designs are making this less of a problem though.

The Fluidized Bed has made the best quality artificial Biological filtration available to the average hobbyist. It is truly remarkable, and most people, including myself, highly recommend it. It has many uses in freshwater as well, and I would use one on any Tanganyikan or South American Cichlid tank for example. We hope this has shown you some of the arguments as to why these filters are so good.

Any questions can be addressed to witmer26@erols.com
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