Maybe I'm confused, but they are designed to overflow - into the aquarium. If you are indicating that they are spilling out over the back of the filter and not into the aquarium, then the mounting to the aquarium might just need to be adjusted. As I recall on the aquaclear (never used a whisper), there is a piece of plastic that can be cut to adjust the angle at which the filter leans forward into the tank. If it is not long enough, the filter will spill water out the back of the filter. You can always improvise by making a little prop to fit between the back of the tank and the bottom of the filter box that effectively causes the filter to lean forward. You could use anything for the prop such as a piece of 3/4" tubing or a straw. All you need to do is make sure the filter is angled forward enough to spill into the tank.
I may have missed the point of your post, and if it is spilling do to the filter pad that you placed at the exit of the filter, you might just have to remove that and keep your water level high enough to prevent the turbulence.
In that case dont worry. Over time the motor may die down a little bit and not output quiet as much which in your case wouldnt really be a negative right. What do you have in the filter for media? I have two foam pads in my mini and ive had no overflow problems at all.
its a mini, i have what it came with in the beginning, the bio rings, carbon and the foam block and then ont the outlet i have a very thin foam pad to decrease the water disturbance. i had it for like one week.
I would try what Rob G. first suggested too, and level the thing so if anything it's slanting toward the tank or level.
Some who have had trouble like that (especially the sponge coming up) have had success with weighing it down with something like a rock.
Bio rings, im assuming they are just there for biological filteration. I recently picked up some bio zorb or something of that nature for my aquaclear and its pretty much the same thing as a sponge (area for bacteria to establish). As i said before and simpte, remove the carbon.
Good suggestion. Those carbon packs that come with many power filters should never be used. They generally remove chemicals from the water that are beneficial to your plants. If you remove this, there should be enough extra space so that you won't see the overflow. If it still occurs, well, there's something wrong with it, because it shouldn't do that...