Point well taken Byron. Now the constant introduction of new and fresh water via consistent water changes, has proven to be the cornerstone of good fish husbandry for discus in particular, and all other fish in general. That being said, the fluval fx5 provides adequate mechanical, as well as biological filteration. But in an unplanted tank, wouldn't a planted refugium/sump help with nitrates? ( something that the fluval alone cannot do? So in the event that you "miss" a scheduled water change, the fudge would provide a bit of redundancy from nitrate build up. So in this case would that still in effect be considered over filtration, or more an alternative method to cover as many bases as possible? Also regarding the hardness of the water and such, what do you consider to be the MOST ACCURATE testing kit on the market? Thanks in advance!
Last question first, I use the API liquid tests, as many here do. I am told the Sera is perhaps better, but more expensive. I have never had issues with the API. Others may be as good, but cost wise they are not cheaper so go with API.
If missing a water change causes a significant rise in nitrates, something is wrong. The system should be better balanced to handle this, up to a point. I never miss the weekly changes on my tanks, except perhaps by a day or two at most, which only occurs if I am incapacitated, and that is not often fortunately.
I've never used refugium/sumps, nor do I know much about them. I always have live plants in my tanks. Let's talk about an aquascape for discus for a moment.
I would always have a substrate. First, the fish expect it (a glass floor is reflective and can stress out any fish, this is not at all natural). Second, there is a complex host of bacteria (beyond nitrifying) that live (or should) in the substrate, further helping with keeping the water chemistry stable.
You could have a planted tank. This is simple, straight-forward, and less expensive than complicated filters. A good canister rated to the tank size would suffice with this. This is how I run my tanks, and I would readily add discus to a couple of them as they are setup now, if I decided to have discus. And I wouldn't change my routine of weekly water changes (half the tank). Here is a link to a thread from a year ago, with a photo of Geomancer's 125g tank and one of my Amazon tanks, both of which would be good for discus:
http://www.tropicalfishkeeping.com/aquarium-plants/125-gallon-densely-planted-discus-aquarium-123275/
Another option is more of a biotope setup. Sand, chunks of wood and branches, but no lower plants. But a thick layer of floating plants. The discus would be in heaven, as they do not like bright overhead light, they expect cover above them, and and the floating plants remove a lot of nutrients including ammonia from the water. Here's a photo I found online of a typical Amazonian aquascape, and this would be ideal. I would have more floating plants, but they will fill in (Amazon Frogbit is shown in this photo].