Researched and Formatted by: Juanita Hoffman
Psychological Benefits:
Several types of freshwater substrates are great appearing. There is a wide range of size, shininess, and color to choose from. Basic choices are natural or not, the latter often being artificially colored then epoxy coated, and therefore chemically inert. Natural gravels encompass flints, silicates, dolomites/coral sand/shell materials, and carbonaceous rock of various types, even crushed (and hopefully tumbled) glass. Other than consideration as to appearances, natural gravels can be useful adjuncts to maintaining good water quality, slowly dissolving over time.
The more broadly spherical the pieces of the substrate, the more consistent the flow, and therefore less channeling and packing-down. The complete reverse is found in more flat offerings (like silica sand) that lay-down amongst each other, effectively clogging-off water circulation. Our favorite mental demonstration of this principle is the comparison of a bed of poker chips versus one of marbles. Get it? Go with the marbleshapes. Sharp edged substrates can cause other troubles with freshwater livestock, cutting into the skin of burrowing fishes, even snipping off the barbels of scavenging catfishes.
As a rule of thumb about two gallons a minute per square foot of filter/tank surface area (2 gsfm) is about right in terms of flow through the filter bed. There is no practical upside limit, i. e. three gallons per square foot per minute would be even better. For smaller systems you can get this flow rate with large airlifts outfitted with (1 mm bubble) uniform (air)stones, or powerheads. For big to humongous tanks you'll want to employ a fluid-moving pump system, possibly in conjunction with an outside power filter rig. The preference here is to run the circulation "reverse-flow", up through the filter plates after the water has passed through the particulate and chemical filtrants of the outside filter. Reverse flow setups provide much better for removal of sediment that otherwise collects under filter plates.
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