Yes, this is or should not be at all necessary, as the fish one would keep in a 20 gallon do not need this much current, and in any case it is not going to have any benefit to water quality.
This is another myth in this hobby. Many do not understand "filtration."
Filtration has two components, water movement and water quality. The movement is the current, and this absolutely must be suitable for the fish species, as different species have differing needs when it comes to the current they live in. This aspect is fairly easy to understand.
The quality aspect is more involved. There is water clarity and water quality, two very different aspects. The filter should serve to help provide clear water, and it does this by forcing the water through media such as sponges, foam, and/or floss. This is termed mechanical filtration. The number of fish and their size along with feeding, live plants, substrate, etc, all factor in to water clarity, so here again, the filter cannot work miracles if things are not right to begin with. Many fish do not really care much about the clarity, so this aspect is primarily for the benefit of the aquarist. It must be remembered that crystal clear water does not mean the water is actually clean for the fish.
Clean water means the water quality. But there are some things filters cannot remove or process, which is why we do water changes. So more filters is of no benefit here. That leaves us with the aspects a filter can handle, if it is suited to the tank volume and the fish load is in balance. Nitrification is the primary purpose of the filter, which is why filters are so important in cycling. The nitrifying bacteria live in the filter--though they also colonize many other areas, such as in the substrate--and the water flow pushes the tank water through the filter so the bacteria can remove the ammonia and nitrite. This is termed the biological filtration. But it has boundaries. If the water flow through the filter is too fast, the bacteria cannot do their job. And at the same time, even if the flow is OK, the bacteria will only reproduce to numbers sufficient to handle the "food" (ammonia, nitrite). So if this occurs in one filter, adding a second or a third has absolutely no benefit. This is where "over filtration" becomes detrimental, not beneficial, either because the filter is too large and moving water too fast, or there are too many filters doing nothing but creating more water current which may not be beneficial for the fish in the first place.
There is also chemical filtration, where substances such as carbon are used to alter the properties of the water. This too is limited, and in some cases may not bee needed, or may bee detrimental by removing nutrients plants need, etc.
