by Dr. Adrian Lawler
(retired) Aquarium Supervisor (l984-l99
J. L. Scott Aquarium Biloxi, Ms 39530
Original to Aquarticles
Diatoms are one-celled aquatic plants that can utilize soluble silica from the water to make their protective opaline or hydrous silica cell walls or shells. When the diatoms die, their cell walls sink and accumulate in beds of sediment; the harvested sediment can be called diatomite or diatomaceous earth. There are about 16,000 species of diatoms in the world.
The shells, or parts of shells, have many pores that give diatomite its valuable properties. The pores are 1-3 microns in diameter; some species have a finer network of secondary pores of about 0.5 micron diameter within the primary pores.
The unique pore structure of the diatom cell wall plus chemical stability make diatomite very valuable in filtering processes. Over half of the world diatomite production is presently used in filtering to clarify wine, beer, and fruit juices, and in private and public water purification systems. Some of the other many uses (from toothpaste to dynamite, pest control, paints, chemicals, insulation, animal feeds, etc.) of diatomite are discussed at: http://en.wikipedia.org/wiki/Diatomaceous_earth
Since about 90% of its volume is composed of open spaces and pores, diatomite has excellent permeability and filtration capacity, and can remove particles of about 0.5 micron from liquids. Diatomite is made into a powder and is used as a precoat on filter septa, filter bags, or membranes. Once the precoat is formed a liquid can be filtered fairly quickly to a high clarity.
Large diatom filters, also known as diatomaceous earth (DE) or swimming pool/spa filters, are one of the best types of support equipment one can use in aquariums and aquaculture facilities.
In the early l980's I found that diatom filters easily filtered out free-swimming dinospores of Amyloodinium ocellatum, and in my outreach advisory program passed this knowledge on to various aquariums and aquaculture ventures around the country, enabling various facilities to exhibit dinoflagellate-free fish, and to raise redfish, cobia, red snapper, speckled trout, pompano, and other species without worry about deaths from A. ocellatum.
At the Scott Aquarium I used large diatom (swimming pool) filters to effectively control, or eliminate, some bacteria, dinoflagellates, monogenea, copepods, etc. Such filters remove many free-swimming infective stages of various parasites from the tank water before they can attach to their hosts, and when the attached adult parasites on the fish die, the fish are eventually left parasite-free; those parasites that have no free-swimming infective stage, i.e., those parasites that reproduce on, and stay on, the same host are not normally in the water column (off their host) and thus not removed by diatom filtration. Diatom filters are usually charged (filled) with diatom powder to filter out suspended particulate matter. When the water discharge from the outflow hose becomes noticeably slowed, it is time to either recharge the filter (after cleaning) with new powder, or "bump" (diatom powder + debris cake is dislodged from filter bags and then allowed to re-coat filter bags) the filter. If the discharge is cloudy, or the tank does not clear up, suspect a hole in the filter fabric that lets diatom powder pass to the discharge side of the filter fabric and into the tank. If the hole can be found, clean and dry thoroughly, and seal it with silicone cement or another non-toxic sealant.
Diatom filters were also charged with diatom powder plus fine activated charcoal at the Scott Aquarium in order to combat free-living toxic algae. The diatom powder filtered the toxic algae out of tank water, and then the charcoal took up the toxins released by the algae. For charging, mix the diatom powder (and charcoal, if used) in a bucket with water. Then pour suspension into the filter canister or suck it up from bucket via the intake hose (while pump is running). Stir filter material in the bucket (if being sucked into the filter canister) to keep diatom powder and charcoal suspended in order to get a fairly even coating on the feed side of the filter when the filter pump is turned on. Add water slowly to any container holding diatom powder so a sudden rush of water does not thrust powder into the air, where it can be inhaled.
For large tanks, or tanks with heavy loads of animals or heavy feeding, it is best to have rapid sand filters also in operation in order to remove larger particulate matter so the diatom filters do not get clogged so fast and need frequent cleaning and re-charging.
One will need to make an intake manifold to attach to the intake hose. Such a manifold is needed so fish, etc., are not sucked up by the intake hose suction. I used PVC pipe about one foot or greater long of a diameter similar to or greater (and adapted to fit inside the intake hose collar) than the inside diameter of the filter hose collar and that had numerous quarter-inch holes drilled in it and one end capped. The uncapped end of the PVC manifold pipe is pushed inside the filter hose collar and secured with a stainless steel hose clamp.
In using diatom filters for many years I observed, on more than one occasion, wounds on injured fish (from collection injury or fights) healed up fairly quickly when a diatom filter was used on the tank. It is assumed that the diatom filter removed bacteria from the water (less bacteria in the water = less chance for bacterial infection) that could have led to wound infection. Some of the fish would position themselves (head first) for varying amounts of time in the stream of filtered water from the filter outflow, which could result in their wounds being washed out, bacteria being washed off fish, etc. Fish are probably smarter than we think.
Diatom filters can be used to reduce, or eliminate, infective agents from being spread from tanks via aerosols, and to eliminate suspended particulate matter that might be irritants to tank occupants. If one has more tanks than diatom filters, rotate diatom filter use on your tanks so that there is no explosive build-up of parasites or toxic algae in any of your tanks. In order to be safe, assume the diatom filter might carry problem organisms to the next tank to be filtered. Thus, the diatom filter should be rinsed, and bleached, after use in each tank so that any living bacteria, parasites, or algae are not transferred to the next tank to be filtered. Or, at least the intake hose (containing any fluid that has NOT gone through the diatom filter and that may enter the next tank) should be sterilized before use in another tank.
After the parasite or toxic algae problem has been solved, schedule diatom filter use on your tanks on a weekly basis (if not enough filters for all tanks), or keep a diatom filter running on a problem tank so there is no explosive build-up of parasites or toxic algae again.