Welcome to FishTankForums Loman!
Yes filtration is one of the many broad and confusing areas, like lighting, in marine tanks.
Basically I would break down filtration into three varieties of two different catagories: (bear with me if you know this already so that I can go through it for the sake of others.)
In
Catagory 1 we have
Natural filtration that uses no manufactured parts but rather bacteria, micro-organisms, and livestock to clean up and maintain detritus removal and nitrogren export.
In
Catagory 2 we have
Artificial filtration that employs the use of mechanical and chemical means to clean and clarify the water column. Nitrogen export is possible through these artificial means as well, although it requires frequent replacements of filtration media to be effective to any great degree.
Deep Sand Beds, Refugiums, and Live Rock all fall under Catagory 1, while Hang On Back, Cannister, and Wet-Dry filters, along with Skimmers, UV Sterilizers, and Fluidized Bed filters all fall under Catagory 2. Although some of those from the second catagory may use natural means to effect their usefulness they still rely on a mechanical medium that will need to be periodically exchanged to maximize that effectiveness, and is therefore considered by me to fall under Catagory 2.
All of these types of filtration with the exception of the refugium can take place either within or outside of the tank. This is where the sump comes into play. All the sump is is an external container which provides the ideal place to house and regulate the different systems. They also add the benefit of increased system water volume.
For instance, if you have a sump that contains nothing but water, heaters, skimmer, and a filter pad to collect detritus then you have a Catagory 2 sump. Wet-Dries fall under this catagory for the same reason even though they generally contain bio-bale type media to promote bacterial colonization. This is because the job of filtration falls mostly onto the mechanical components with the natural ones only providing a little polishing.
On the flip side, if you have a sump that employs a large DSB that equals or greaters your main tank volume where water flow is slowest in the whole system then you have a Catagory 1 sump. Even if you had a skimmer, filter pads, and UV sterilizer integrated into the system it would still be a Catagory 1 because the majority of fitration would be taking place by natural means while the others only do the polishing.
The overall goal of all of the above is to remove free-floating organic material from the water, convert ammonia into nitrogen and remove it along with any other unwanted or excess nutrients, and cut down on as much TDS as possible.
As long as you have the means to achieve those goals effectively in your size system with its bioload then you can use any combination of the above that suits your taste and standards. All you need to know is what each does, or can do.
HOB Filters and
Cannister Filters can perform all the needed jobs to one degree or another of effectiveness. They both have interchangable media that you can exchange and configure however you want. There are filter pads for trapping floating debris and detritus (
mechanical), Carbon and resin type filter pads to ad/absorb nutrients from out of the water (
chemical), and porous or filter type media to hold nitrifying bacterial colonies (
biological).
These systems rely on pulling the water through each filter in a staged succession to maximize the effect of each filter. (ie. Filter Pad > Activated Carbon > Bio Media (Trap Debris > Pull out excess nutrients > Polish out remaining excess)) Flow rates in these systems are usually high and are not ideal for effective biological filtration because the bacteria need more contact time with the water in order to effectively get all the ammonia and nitrites.
Wet-Dry Filters are basically sumps with built-in filtration media (
mechanical,
biological). They allow the water to fall through the filter pads and pass through the bio-media to be filtered by bacteria but they also add aeration to the water while doing so. The water is then collected in the container portion to await transport back to the tank. This container is usually provided as a storage area for heaters and skimmers to heat and polish the water before it makes its way back to the tank.
UV Sterilizers use short wave ultra violet light to sterlize the water by killing free floating bacteria, viruses, and parasites. The key to these systems is light exposure time which equates to flow rate. Move the water slower through the sterilizer and you will increase the exposure time as it passes under the lights. Since UV sterilizers do not release anything into the water that carries into the tank they can do nothing to help clean the tank environment itself, only what is floating through the water column and can be passed through the UV system.
Skimmers mix water in a chamber with billions of microbubbles which will carry any organic solids they attach to up and out of the water to be collected in a cup. Thus they remove organic waste before it has a chance to dissolve and break down into ammonia. Just as with a UV systems light, the skimmers bubbles need as much contact time with the water as is possible into order to effect the best results. The more time the bubbles have with the water the more chance of attaching as much debris as possible for export. Flow rate is once again a determining factor, but this time it is one that will need to be balanced with volume. More volume allows for slower flow rates.
Refugiums are much like Deep Sand Beds and are a topic for lengthy discussion. The basic idea is that they provide a large surface area for the colonization of nitrifying bacteria and detritus consuming organisms and allows for slower water flow to maximize the contact time. They add the benefit of greatly increased water volumes in some setups. Some systems have small HOB mini-refugiums. These mini-fuges are more effective for the propogation of pods as a food source than for nutrient export although they do help with the latter.
Fluidized Bed Filters from what I can tell are much like cannister filters that pass water through a substrate material in an effort to combine both
mechanical and
biological filtration into one. I have never used one of these myself and will need to do a little more research before I can explain their use any more than that. I mention it simply because it is another form of filtration out there that you may come across.
I am sure there are a few things I missed here so if you think of anything just ask. I am getting ready to go see my 5-day-old niece and need to cut this short.
HTH,
Aaron
It is more extensive than most people i have talked with. I hope you enjoyed the visit with the niece. Babies are always such fun. Thanks for the welcome - last night I was so focused on the question I neglected all the pleasantries. I am really impressed with this forum and the amount of traffic and quality of answers. It seems to offer a good deal more than other alternatives.
I do go on at length sometimes. Thank you, I always enjoy seeing my nephews and nieces, but my new baby niece is more of a joy as is always the case with babies. 
