Properly designed filtration systems can perform multiple functions that all play a roll in proper water management and aesthetics. Of these functions, efficient aeration, nutrient export and the breaking down of organic waste are top priority. The ease of maintenance to the filtration system is a close second.
A good filtration system first efficiently traps dirt / debris floating around in the water (mechanical filtration). Having decent water flow / current in the aquarium helps keep debris suspended to give the filter a better chance of catching the debris. What ever devise is the primary dirt trap needs to be easily accessed for frequent / at least weekly cleaning. This is a demonstration of nutrient export. What is meant by this is instead of leaving organic debris in your primary dirt trap and letting it decay for several days or worse, weeks, you rinse the debris down a sink drain, effectively removing the waste from the system and removing the negative impact the organic matter would otherwise have on the system. It is false to assume that the aquarium is clean because the "dirt" is in my filter. The Dirt" is still in your water and is still decaying and having a negative impact on the water quality.
After water has been mechanically filtered, the particle free water should then be delivered to the part of the filtration that performs biological filtration, in other words, the part of the filtration that consumes and/or breaks down dissolved waste in the water. This function can take place by numerous environments such as Algae, remote deep sand bed, refugium, & wet dry filter to mention a few. In this case, being a fish tank, a wet dry type filtration would be a very good option. Wet dry filters are essentially a mini sewage treatment plant. Countless populations of bacteria feed off the waste and convert harmful ammonia & nitrite into less harmful nitrate. What we need to do as the aquarium owner is provide the bacteria an enormous amount of surface area for the bacteria to live, and force pre-filtered (particle / debris free) aquarium water to that part of the filter multiple times per hour. Doing so allows the bacteria to feed off of and break down the harmful waste products in our water that could otherwise kill our fish. Nitrifying bacteria consume a lot of oxygen, this is partially why better designed wet dry filters have the media (typically bio balls) up out of the water with aquarium water trickling down through them. The bacteria function more efficiently with the higher content of oxygen available to them up out of the water. THe reason bioballs are used is because they are designed to have a huge amount of surface area but take up small a volume of area. You should have approximately 1 cubic gallon of bio balls for approx 20 gallons of aquarium water. A tip, Putting a simple air stone under your bio balls can more so increase the efficiency of the filtration system by supplying more fresh air to the bacteria.
Another desirable side effect of a wet dry filtration system is they can very efficiently degass (oxygenate) your water. All the crashing and churning of the water through the bio ball gets co2 out and oxygen in, even more so with the airstone below the bio balls.
Chemical filtration can also take place before or after the bio balls. This typically take place in the form of activated carbon or media designed to absorb phosphates or nitrates, etc.
Protein skimming also can be incorporated into a wet dry or sump type filtration. Ideally, the protein skimmer should be supplied with raw unfiltered or pre filtered water before the biological portion of the filter but is is acceptable to have the protein skimmer after biological filtering. A positive thing about having the protein skimmer in the sump is you can have better control over fine bubbles in the water. Hang on the aquarium type Protein skimmers often put tiny bubbles into the aquarium that can make the water foggy looking and could irritate fish / corals, etc.
To get to the point of the topic of this post, Sump style versus using a canister. 1st, canister filters are not easy, convenient filters to work on. You really need to clean, rinse or replace your dirt trap at least weekly. This is a major chore on a canister filter and could be expensive. If it is difficult to do, your eventually going to get sick of messing with it and it won't get done as often as it should be. If you had a pre filter box with sponge filters in it trapping debris, it is very simple to remove the sponge, take it to the sink and rinse it out. And, pre filter sponges typically last many, many months before needing replaced. If it's easy and convenient, your more likely to do it often. Another benifit of a prefilter box is they surface skim the aquarium. This is very desirable because the surface of the water typically has the highest nutrient concentration. Delivering this to the filter, especially the protein skimmer can make a noticeable improvement in water quality.
Next would be adequate surface area for nitrifying bacteria & having high levels of oxygen present for the bacteria to perform efficiently. Canisters typically have a small area dedicated to biological filtration and are not and oxygen rich environment. They often yield unstable to poor water quality on the typically stocked fish aquarium.
Properly designed wet dry / sump type filters are superior to canisters filters on many levels. Fighting with an aquarium is no fun and can ruin the whole experience not to mention get expensive. Most people with canisters do not have long term positive experience with them. This early in the setting up of your tank, you could save yourself a lot of aggravation by starting with a better, easier to maintain and cheaper to maintain system.
Hope this helps, if you got more questions, ask away.
Joel
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