I have a Hydrosponge filter in my betta tank because it filters and aerates the water without producing any current. The Hydrosponge you saw listed on e-bay should be sufficient for a 75 gal
- depending on what kind of fish you have. If your fish like a current, you would have to get a powerhead.
The big downside is that you have to clean sponges more often than HOB
filters and you have to be careful about how you do this because the good bacteria live in the sponge. To clean them, you just swish them around in a bucket of dechlorinated tank water. Also, you need to gravel vac more often because the filter doesn't trap large debris effectively. I wouldn't recommend this filter for messy fish like cichlids.
The other downside is that it's kind of ugly, but you can hide it behind plants.
Finally, it is driven by an air pump that must run 24/7. Quiet air pumps are quite expensive and I doubt if the pump in the ebay auction is quiet at all. Something to think about if the tank is located someplace where you like to read or sleep. HTH