I'm not in agreement with everything stated on this thread and would like to offer some additional input.
The reason I asked to very specifically describe what you do to your Fluval & how often was because it was my suspicion that what you were doing (or better stated - not doing) was a major cause for the nitrate issue. Servicing this filter only once per month easily could be the majority of the problem. Regardless of what type of filtration system is being used, the dirt traps need to be cleaned minimally once per week, more often is even better. Leaving the debris in the sponge filters in your fluval for a month at a time allows the matter to accumulate and be broken down into nitrates. Cleaning the sponges at least once per week to remove the debris will significantly decrease the waste load on the system and take away from the aquarium some of what would eventually become nitrates. If you will recall, this exact subject was covered extensively at the March reef club meeting. The importance of nutrient export was a major part of the meeting topic. To recap, Nutrient export is a term that means " the physical removal of organic material" versus letting bacteria brake down the organic material into nitrates. In other words, if you take the dirt out of your filters / water, it's no longer there to brake down and eventually become nitrate as well as cause other problems.
Although canisters may have their place, I strongly advise against their use as the only or primary means of filtration. They are not convenient to service (you want to take it apart and clean it once per week or more?) & They have way to small of an area dedicated to biological filtration. These two reason alone should be enough reason to avoid this type of filter. Who ever suggested that it only needs cleaned once per month is ignorant, where do they think the waste goes? The negative of a canister so out weighs the potential benefits that I see no valid reason to use one.
On the subject of Nutrient export, it is one of the most important functions taking place in our aquariums. It doesn't matter if it's fresh or salt, fish or reef tank, the removal of waste is very important. I don't completly agree with the prior opinion on types of nutrient export though. A protein skimmer is an excellent example of nutrient export, it physically removes waste from the aquariums water. The earlier mentioned practice of regularly cleaning out your pre filters is another example of nutrient export, this is the physical removal of organic matter from your filtration system. In filtration systems that incorporate plants or algae (vegative filtration) is another example of nutrient export. The plants & algae grow by using the nutrients in the aquarium water. As these plants & algae grow and bank nutrients in their tissue, we harvest them - remove portions of them from the filter - throw them in the trash. We essentially are throwing away (exporting) nutrients when we discard the excess macro algae. Water changes are a form a of export as well.
I do not agree that live sand, live rock & clean up crew are examples of nutrient export - they simply are not. These are examples of living organisms that utilize organic waste and convert it into more broken down organic waste. None of these examples eliminate or remove waste from our system.
With your current set up, you need to do water changes more often & clean your pre filters at least weekly. An RO/Di is a good idea but is not the solution to your nitrates, better maintenance practice and more frequent water changes is the solution.
Getting a better filtration system all together would be ideal, depending on your goals a wet / dry - protein skimmer combo would be good for a fish system, a remote deep sand bed w/ a protein skimmer is a great option for a reef type system
Joel