Okay, I get no commission, but let me defend the lowly HOB in the face of all these "canister snobs"
There is no waterfall with an AC HOB unless your water level is low. Why have a low level in your tank. Water is inexpensive and another half inch or so solves the problem. My AC's have a lip to direct water across the tank surface. If the water is at or above this lip, there is no waterfall.
There is a return flow, but there is with any filter that moves water. In the case of the AC's the flow is across nearly the entire length of the filter. I minimize/eliminate the flow with a simple water bottle baffle (with holes made with a soldering iron) that cost me nothing except the water I drank from the bottle.
The HOB emptied and the motor burned out when there was a power failure? The only way the filter compartment can drain is if the tank level drops 6-8" or more causing the water to siphon back into the tank. This doesn't happen when the power goes out! I turn my filters off all the time when feeding and the HOBs just sit there full of water until the power comes back on and then they continue like nothing had happened.
Someone complained about seeing it on the back. Well most tanks are against a wall - I use a background so I don't see the back wall (or the filters and cords and such!).
I even saw where someone (in another thread) claimed that his AC filter media plugged and the tank was emptied all over the floor. I've blocked the flow on the outlet side of my AC filters and the water returns to the tank on the inlet side.
I don't where this stuff comes from?
One of my AC 70 HOB filters contains three (3) AC 70 sponges (no other media) making it an exceptional external sponge filter. It is set to low flow, maximum re-filtration.
My other AC 70 HOB filter, also set to low flow, is full to the brim with Seachem Matrix bio-media. It uses a Fluval pre-filter sponge wrapped in nylon stocking material on the end of the inlet tube to pre-filter and further reduce/slow the flow through the bio-media.
These are working great for me and the combined cost is less than the cheapest canister filter and maintenance is much easier since I can service the media without removing the filter or even turning it off if I choose not to.
Now, not all HOBs are created equally. And don't get me wrong, canisters are great and so are sumps when done correctly - but more for larger tanks. For small and mid size tanks, HOBs are efficient, cost effective and easier to maintain.
So, ladies and gentlemen of the jury, lets be careful and consider that the lowly HOB has been unjustly accused and discriminated against in favor of pricey Alpha Romeo can filters and even overly priced submersible eyesores.
The defense rests.