Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Depends on the manufacturer. Read the label. Some test with the filter filled with media, so their numbers are not too far off. Some test when the filter is empty, and are just imaginary numbers with no basis in reality.
Remember that as the filter fills with debris it will slow down.
I generally aim for 10x, combination of filters and power heads. I like the larger volume of most canisters. I like the large volume and the layout of the media in the Aquaclears.
If I was keeping Golds in a 40 I would use at least a Rena Filstar XP3, yes, a THREE. Or else an Aquaclear 70. That is a LOT of water movement, but it can be arranged so the Golds are not blown all over the tank.
I had some golds in a 45 gallon. They grew so fast they were only in there for a few months, then out to the pond. They were Shubunkins with long fins, so highly active fish, though not quite as fast as a short finned comet.