For my fresh and brackish tanks i just use HOB filters especially a bio-wheel. I looked into using a sump but didnt seem to be as beneficial as with salt
For the water i switch my DI off and go with RO but even then youll have to add stuff to get the water right for fresh, i dont have discus so not positive but i think they require a few more additives if you use RO. And honestly my challenges are with saltwater, fresh seems to be more forgiving, the only thing ive had to worry with fresh is some initial die off with plants as they are added and those nasty snails that will hop on them. I setup a fresh planted for my nephew, whos 8, and its pretty stocked with plants but havent had to dose or add any CO2 for him and in 6 months he hasnt had a piece of algae. With my brackish its also stacked but i did dose and have a small DIY CO2 system i use sometimes to push growth. Now with the 36 bracksih i did have big algae outbreak till i trimmed the plants back and poof its gone. Got some really nice plants from Pollys and Gabe, and yes i feel stupid but bought one from petsmart which then died and gave me 5 bad snails

If you do find those snails call me and ill take them, my puffer loves nasty lil snails lol