On a 55g planted tank, either a canister rated for that size tank or a sponge filter (also rated for the tank size) would work. On the sponge filters, Hydro make them in several sizes including one for a 55g.
To pick up on the "too much filtration" point, yes, it is possible to over-filter, especially in planted tanks. But there are too separate but related issues to this, biological filtration and water movement (current). Both are important.
Encouraging biological filtration can be detrimental to plants. Plants need nitrogen, and they prefer it as ammonium. Ammonium comes from ammonia produced by the fish and bacteria--and with the bacteria we are talking about the aerobic bacteria that live in the substrate as opposed to the nitrifying bacteria. Ammonia is toxic, but in acidic water it changes automatically into the basically harmless ammonium. Plants grab ammonium. In basic water (pH above 7) plants grab the ammonia and change it to ammonium and then assimilate it as their preferred source of nitrogen. They are faster at doing this than nitrosomonas bacteria, but encouraging biological filtration allows more nitrosomonas bacteria to develop which compete with the plants. Unless the tank is overstocked with fish, sufficient plants will easily handle the ammonia/ammonium. Thus, encouraging nitrifying bacteria is counter-productive.
With respect to the water movement or current, this also can be detrimental to plant growth. Water circulation is important for bringing nutrients to the plant leaves and roots. And it also removes suspended particulate matter by trapping it in the filter, thus preventing it from building up on plant leaves. Too much movement can cause nutrients such as CO2 to be pushed past the plants faster than they can assimilate them, and in the case of CO2 this often results in the CO2 dissipating out of the water so it is lost. |And when CO2 dissipates, oxygen enters. And excess oxygen is also detrimental because oxygen binds with many nutrients, such as iron, making them unavailable to plants. So too much water movement is detrimental to plants on several levels.
As filtration drives both biological filtration and water movement, you can see fro the above how it can be "too much" in planted tanks. A sponge filter is ideal. And a canister that does not have too great a flow, which can be directed against the end wall to create a gentle current down the tank, is also ideal. Of the two, the choice might depend now on the fish in the tank, as some prefer little or not current, some need a current.
Most forest fish (characins, rasbora, dwarf cichlids, angels, discus, gourami) come from waters with little movement. Some catfish and loaches occur in streams with more current. The filtration must take into account what fish are going in the tank, since either group can suffer if what they require is lacking. If loaches and catfish are planned, a canister works best as you can have some current at one end of the tank that will dissipate as it moves down the tank, and the current-loving fish will naturally take up residence near the filter outflow. Many forest fish will take up residence at the opposite end, farthest from the stronger current. This occurs in my 115g Amazon riverscape aquarium. The cardinals and rummynose, quiet water fish, remain in the right half of the 5-foot tank, farthest from the stronger current. At the left side, the Woodcats, pleco, whiptail and a few cory species stay there,along with the Pristella Tetra. By providing both options, which is possible in a 4-5 foot tank, both fish groups are getting what they need to be less stressed and thus healthier.