sorry HOB == hang on back

the typical filter that sits on the back of the tank with a tube that sucks water from the
tank and usually returns it through some kind of waterfall
pinguins,aquaclears, tetra has one, there simple, usually complete filtration
(mechanical,chemical,biological) , generally good for FW tanks, there use in SW is
specific to what kind of tank your keeping or interested in keeping.

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richard reynolds