When talking about aquarium filtration, you are actually addressing 3 things, mechanical, biological and chemical filtration. Mechanical will remove detritus (waste) eg; excess food, fish waste etc. Keep in mind a mechanical filter is only as good as what it sees. SO if you have more circulation or dead spots in your tank, that fish waste will stay visible in your tank causing you to do frequent PWC's.
Chemical filtration is consistent mainly of activated Carbon to clear the water. That should be changed very often as it can RE-RELEASE what is absorbed eventually. However, a protein skimmer is designed as a chemical filter, removing dissolved waste and organic compounds from the water by foam fractionation (putting air into the water stream). The waste, or 'skimmate' rises into a collection cup at the top of the skimmer and you simply empty the cup and replace. For a 55gal there are plenty of good hang on skimmers that will be very effective. CPR's Bak-PAk is an excellent and affordable choice.
Biological filtration is mainly consistent of a means or place for nitrifying bacteria to grow.. Live Rock, Bio balls etc... Billions, maybe trillions of aerobic and anaerobic bacteria live on this 'bed' and and work to break down the harmful ammonia and nitrites that accumulate in a tank into less harmful nitrates, (unless you have a reef tank).
The Fluval FX5 or Eheim pro 3 are canister filters and can accommodate all 3 methods of needed filtration in one spot. I do not use these in my systems only because i opt for a different method (which i'll explain briefly). However, either one of those filters is an excellent choice, with the Fluval being slightly more affordable and very comparable. Those filters are extremely quiet and usually need to be felt or listened to very closely just to heat them. Protein skimmers are not included on either one of those filters or any canister filter for that matter.
I opt for the full on wet/dry and additional sump method. I did not drill my tank so i have (2) overlows with micron filters (mechanical) added that drain water into a trickle filter, pouring water over a bunch of biological media (biological) creating the nitrifying bacteria i need. from there the water flows into another micron sock (mechanical) into a sump where i have a protein skimmer (chemical) some additional live rock (biological). I do have a dedicated pump in the sump pushing water through a UV sterilizer (chemical) and back into the sump as well. From there at the other end is a return pump to push the filtered water back up into the tank to start the process all over again. This happens in my tank at 3000gph.
That may sound like overkill to some.... but my fish are NEVER sick, my water is always clear and my nitrite and ammonia levels are always 0 ppm. Plus, i can keep anything i want in my tank. Thats my 225gal. In my 150gal I use a hang on protein skimmer, a submersible 13w UV sterilizer and (2) Eheim 2213's plus an Eheim 2217. I have no troubles in that tank either but i only have 3 fish in it.
In short your choices will be canister filters or trickle filters (wet/dry), some people use sponge filters too. Whatever your choice, your filter MUST address all three of the requirements, mechanical, biological and chemical for your tank to thrive.
Hope that helps.
