07-19-2012, 08:31 AM
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Those guidelines you found, as another poster mentioned, is for GPH (Gallons Per Hour) and not the aquarium size.
In all my tanks over the years (7 tanks) I have always just gone with a filter rated in size for my aquarium. I had four different 10 gallon tanks, all using the cheapo Topfin 10 HOBs that they came with. A 29 gallon with an Aquaclear 30, a 20 gallon with the cheapo Topin 20, and my 125 gallon with an Aquatop CF-400UV (which is rated for 125 gallons).
All my tanks have been your garden variety community tanks, so nothing particularly messy, and no large fish.
In essance, X fish will produce Y ammonia which through the nitrogen cycle will result in Z Nitrate. Those numbers are not going to change much, pretty much only will as your fish go from juviniles to adults.
As an example, say you had an established 20 gallon tank with all adult fish (they are no longer growing larger) and a 20 gallon HOB filter. Assume with weekly water changes your tanks water was 0 ammonia, 0 nitrite, and 40 ppm nitrate.
If you then added an additional 60 gallon HOB filter, your water parameters would still be 0 ammonia, 0 nitrite, and 40 ppm nitrate. In other words, that second larger filter would not be giving you any better water. The only time an extra or a larger filter will give you better water parameters are if your current filter does not contain enough bacteria to keep ammonia and nitrite at 0 ppm. This will only be a problem if you overstock the tank.
Some people want the extra water flow because it will kick up more waste off the substrate that then will get sucked into the filter. So the tank will look cleaner, and I stress the word look. All that waste is still in the tank, still being broken down by bacteria.
Depending on the fish though, lots of water flow will cause them stress, most community fish come from ponds or slow moving streams. They are not adapted to raging rivers. On the other side of the spectrum, fast water fish like the Hillstream Loach need a heavy current. Usually that's best obtained through power heads, and not extra filters.