Hydrosponge filters are all you need for biological filtration for feeder guppies. They are the industry standard. Commercial feeder goldfish systems don't use biofilters, because water changes are more efficient, and nitrification doesn't work in the cold temperatures they are kept in, and ammonia is non-toxic in cold water.
Chemicals (Amquel or Prime) are used to remove the toxic ion of ammonia in shipping water, as temperature will climb, but these chemicals aren't cost effective for holding tanks. Zeolites will bind ammonia, but they are quickly expended in crowded holding tanks.
I had a 30,000 gallon wholesale facility for 8 years and ran all of my tanks with huge water changes (via 3 x 750 gallon holding tanks to eliminate chloramine, aerate, and heat to 78F) and Hydrosponge filters. gas exchange is carried out at the air/watyer interface at the surface of the aquarium. Air lift filters efficiently move water from the bottom of the tank to the surface for oxygnation. Power filters don't offer this efficiency.
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Bioball wet/dry systems are great for gas exchange, but lack the surface area and stable conditions offered by sponge filters.
Fluidized bed filters are very compact, but are not a stable environment for bio-films to develop for nitrification. Approximately 80% of the nitrifying bacteria in your system is found on detritus. This doesn't mean you should encourage an accumulation of detritus, but you do need to have a stable environment for it to exist on media, where well oxygenated water can pass by, and nitrifying bacteria can live.
Nitrifying bacteria grows as a film on the detritus, which is in turn, is attached to some form of filter media. It continues to grow, layer upon layer, until it becomes unstable (heavy) and sloughs off. As one area sloughs off, another is proliferating in a constant cycle of growth and die-off. Fluidized bed filters and wet/dry filters cause wash-off, whereby the flow of water causes premature breakdown of mature bio-films. The constant grind of a fluidized beds are therefore a poor home for delicate bio-films to populate.
Biowheel, and sponge filters are the most efficient, stable sites for nitrifying bacteria. Biowheels are better than sponge filters while they are running properly, as they offer greater gas exchange for fish respiration, more oxygen for bacteria, and pre-filtered water to limit the collection of detritus; however, they require electrical motors, frequent cleaning, and the moving parts need servicing to assure the wheel keeps spinning. While sponge filters are slightly less efficient, they are very reliable with little maintenance required and a low operating cost.
The Hydrosponge is the industry standard because they are durable, have a large sponge, move air well for gas exchange at the air/water interface at the surface of the tank, and they have a spacer at the bottom, so it doesn't become too much of a mechanical filter. They are balanced and weighted so they don't fall over easily or float to the surface. Suction cup and box-style sponge filters are problematic, as they tend to fall off the wall and float to the surface where they cease to function.
Hydrosponge filters are not mechanical filters intended to remove POC (particulate organic carbon), they are designed as biological filters. They will collect some particulate matter, so they should be gently squeezed in a bucket of aquarium water to keep the pores open periodically. If they get really dirty, you aren't keeping up with manual siphoning of the tank bottom. They can be thoroughly cleaned if this should happen, but a well established sponge from another tank, should take its' place, or only wash one of the two or more sponges in the tank at a time. Stagger cleaning over weeks or months.
A steady flow of new water will drastically reduce ammonia on its' own. Cooler water has a higher D.O. (dissolved oxygen) rate, and decreases the toxicity of ammonia. Seachem Ammonia Alerts are a good idea for constant monitoring. Higher D.O. will aid in fish respiration, and nitrification.
Nitrate will also stay low, as the water changes will circumvent the nitrification process entirely. Nitrate will not adversely affect fish health in retail holding tanks. Long term exposure to elevated levels of nitrate will inhibit growth, but you are only holding the fish for a week, and you certainly aren't growing them.
As Cayars mentioned earlier, feeder fish are a poor food source for any fish. They have little nutritional value (no EFA's etc.), and are vectors for disease transmission.
Most fish mortality in commercial systems is due to poor fish health. Prophylactic treatments that do not adversely affect water quality, will further reduce casualties. Dead and dying fish decrease water quality, so a trickdown effect causes further losses.
