The 603B has an actual flow of about 45 gph
using the stock media and a 3 foot drop between the filter and tank. Still adequate for a 10 gallon tank. I wasn't happy with it in my 20 gallon. The low flow velocity wasn't moving water (and the CO2
I was injecting) throughout the tank.
I've used a C3 (similar to the C2 but with a bigger impeller and media bay). The filter pump is the same used in the AquaClear line. It moves a sufficient amount of water. The media set up is unnecessarily complicated and not as conducive to using custom media configurations (unlike the AC
). If you get another HOB
filter, I'd suggest getting an AquaClear. The filter will run for years, and you can use cheap DIY media.