There is so much to say about this filter, it's hard to know where to start.
I bought this filter to replace a very ineffective Marineland Pengiun HOB filter. I have a 20 gallon tank, but have a heavy bioload due to a dozen or so fish being born in our tank (thank you Petsmart). The issues we ran into with the Penguin filter were many (see my review), so I looked for a more powerful and configurable filter. The Aquaclear 70 fit the bill.
The housing has plastic walls that I think are a bit thin. I feel the need to be very careful with it when I take it apart and put it back together. That's about the only critical thing I have to say about this filter. It has a dial on the back to help keep the filter level. The impeller and motor can be easily replaced with a simple twist.
You get two intakes with this unit, so you can have it at mid-level, or extend it to get closer to the bottom. The intake swivels a bit, which causes a reduction in the water intake. This is a very useful feature, as it recycles some of the water already in the filter to mix with the water from the intake to give you extra clean water, just at a slower rate. This also comes in handy when you want to reduce the water disturbance for feeding.
The filter media included with the unit is exceptional. You get a filter sponge that catches the debris, a carbon "sock" to catch the very fine particles and chemicals, and "bio max" which is just a media to keep beneficial bacteria. The sponge can be rinsed and reused to your heart's content. It's very sturdy and seems to be effective. The carbon "sock" is huge! Although there is some debate among aquarists if you need continual carbon filtration or not, you certainly can get a lot of chemical filtration should you so desire. The last media, "Bio Max", is rough surfaced cylinders meant to capture bacteria to help keep your tank clean. This is very helpful since, if you were to replace your sponge and carbon, the beneficial bacteria would still reside in the "Bio Max".
Now the true beauty isn't with what you get (which, again, is great), it's what you can CHOOSE to do. The filter housing is very simple, and you can put in most any filter media you want. You can cut filter fabric to size, remove the carbon and add another sponge, or use a different biological filtration method. This, I feel, is key to running a clean tank. If I were to treat the tank with a chemical, I can simply remove the carbon and leave the other filter media in place. This wasn't possible with the Penguin and wound up being a serious problem.
Another great aspect of the filter media is the price. Seeing as how you don't need to replace the sponge or biological media, you just have to replace the carbon "sock". With that, you can buy a replacement for this specific model filter, or you can create your own by buying a filter sock and carton of activated carbon.
We've been running the filter for months without issue... Ok, somehow our Otocinclus got into the filter. Luckily, I found him and got him back into the tank. He must have been watching when we had Finding Nemo on the TV. Maybe an intake with thinner grates would be something to explore.
One thing I would caution against is Hagen's return procedure. I had picked up one of these from Petsmart and it was defective (I'm almost certain it was used and Petsmart put it back on the shelves). Hagen said that if I want to return it to them, I would have to pay for all of the shipping. Luckily, Petsmart exchanged it without issue.
I highly suggest this filter and hope it will work out as well for you as it has for me.