Dunno about the 330, but my 170 and 125's both mention something about a "spillway" designed to alert you that it's time to maintence the filter....
In fact, they have the manual for them on-line upon further looking. Here's the exact wording:
Here's the link to the manuals http://marineland.com/products/manuals/penguin_manual.pdf Page 8
In fact, they have the manual for them on-line upon further looking. Here's the exact wording:
When filtered through a clean cartridge, water returns to the aquarium via the BIO-Wheel Spillway (Fig. 10a). As dirt collects on the filter cartridge, the cartridge gradually becomes clogged and water flow slows.Water begins to f low from the area around the Intake Tube (Fig.10b).When this occurs, the cartridge needs to be replaced. Improper water flow. Time for a cartridge change.
Here's the link to the manuals http://marineland.com/products/manuals/penguin_manual.pdf Page 8