HOBs or "Power Filters" as they are often called in the USA are indeed also a thing you should consider. In fact I believe we've had a number of members with dual AquaClear large HOBs, one at each end of a 55g and this is potentially a very good solution. I'm not sure of which AquaClear models would be right. You'd have to study the flow specifications. You could also go with one external cannister and one HOB, which would give you some experience with both types and enhance your learning.
I mention the AquaClear because it has received high marks from the members for being "flexible." Now what does that mean? It means that the filter box is basically a big wide-open rectangle into which you are left free to decide on and put your own various layers of media if you want to. Of course AquaClear tries to sell you on their own packets of media to fit in but you're not tied down to this. Many other designs are much more aggressive in trying to have the box and cartridge designs force you into only being able to choose media materials from the maker of the filter. Untold engineering man-hours are spent trying to get you back and buying their cartridges rather than designing the filter to provide better filtration for fish!
Winterlily, in any filter planning, the first bit of homework you do is to write down the makes and models that seem in roughly the right range. Then you must search the web sites of the makers trying to find the first bit of data that tells you the "flow rate" for each model. They will bury this info often in odd web pages or PDF files or even only in the user manual for the filter. The most confusing thing is that they will be more prominent in telling you the size tank the filter is recommended for. This is -not- the same thing as the flow rate specification and often makes the filter look better than its flow rate suggests. Even the flow rates themselves are optimistic and one should consider sometimes the next model up to get the rate one really wants.
Recommending flow rates is full of pitfalls. Once again its one of those things where an experienced aquarist can make a tank work with a wide range of flow rates but a beginner can still mess a tank up even with things that are supposed to be the best. At TFF we often recommend a starting point for consideration as being a 5x turnover rate. For your 55g then that would be 275 US gallons per hour, which of course would move all the water in your tank through your filter 5 times in one hour. This sounds like a lot but in fact its a pretty normal looking filter flow once you get it running. The trade-offs with flow rate are that higher flow rates can begin to push your fish around more than you want and you'll be having to find ways to adjust that somewhat so they get some peace. In the planted tank hobby, people often choose to filter all the way up at 10x turnover, or they will use powerheads to compliment the water movement at lower levels of the tank.
In the end, one baseline thing to take away from the above paragraph is to at least make sure you're doing more than 3x turnover. 4x through 10x will all easily help an aquarium to run well. But flow rate is only one parameter out of many. You already know about media volume. That's what creates the "pecking order" among filters, with sumps at the top, external cannisters and a few large HOBS as next best, regular HOBs next, large internals next and small internals last. You can have too much flow rate but you can't have too much media volume... mother nature's media volume is essentially infinite from a fishes point of view.
A lot of what filter shopping is all about are the features that give you more flexibility or the features that make maintenance easier. Big HOBs and cannisters and some large internals can make media placement so easy. In many cannisters you simple lift a tray by its handle and its like a basket, ready to hold whatever you want to put in there.. ceramic rings, ceramic gravel, a big sponge, crushed coral, whatever. The handle lets you dunk the tray in your tank water bucket for cleaning. A big HOB is easy to lift off the tank and its dry as you carry it to the sink. Same for an external cannister. You just shut off its hoses with their shutoff levers, unclip the dual hose block and carry away the cannister. Lifting off the pumphead gives ready access to the impeller, which should be cleaned and maintained at each filter cleaning.
Another thing to consider is the noise factor. Is the tank in a noisy fish room in the back yard? Is it left alone at night in a family room? Is it in the living room where you want it quiet when guests are visiting? Is it in a bedroom where you want the most silence of all? Internal power filters can be very quiet as they are quieted by the tank water! HOBs can be a bit noisy in that they can sound like a little waterfall. High end brands of external cannisters are sometimes bought by experienced aquarists just for their quietness. Sometimes people choose two filters rather than one because the biggest filters sometimes get quite a bit louder than the medium sized ones.
Oh well, this is getting long enough for starters...
(besides I think my cell phone's charged now, so I can go, lol)
~~waterdrop~~
