The Aquatecs are almost identical to Penguin models (Marineland). I still have a couple of Penguin 350's and have modified them so they don't use any disposable cartridges. The option suggested above (just media in a bag, with or without sponges) will work well. Another option is shown below, which is what I have done (sponges inside cartridge frames, glass noodles inside plastic frames, and I do use the biowheels). Regarding the biowheels - some people don't like and and remove them stating they don't work. I disagree, and even if they don't work as biomedia (I don't believe that to be corect), they are great at producing additional oxygenation.
Regarding the filters working for discus - sure they will work, but the media will need to be cleaned very frequently (not a big deal), but large and very frequent water changes are going to be the key, as well as having a well chosen group of fish (due to inevitable hierarchies).
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