I know I don't want Bichirs or the Giant Doradid recommended to me. The first one is ugly (sorry Bichir lovers) and the second one will outgrow my tank. I was told SD's can trigger an attack response in Oscars and I don't want that. I feel like such a Noob. Anyways, I'm looking for fish that originate in slow-medium flowing rivers with lots of wood and rocks that are large enough not to be eaten by the Oscar. Any catfish is welcome, just not too big, and not too nocturnal and shy.Yes your understanding of filtration is correct, and your tank is well filtered. What you are missing I think is that nitrates are removed via water changes. Nitrates are the end product of an efficient, mature biological filter doing its job. Prolonged exposure to high nitrate levels are a severe health hazard to any cichlid, and more so than most for an Oscar. A Sailfin pleco is one of the heaviest bio load fish you can keep. Oscar is up there as well. I have the same tank as you, 2 adult Oscars and a foot long Sailfin plec. The plec makes more mess than both Oscars combined. I do twice weekly water changes of 60-75% per change just to keep nitrates at a less harmful level.I figured the sump/wet-dry would add volume to the tank size and convert everything to nitrates because of how much bio-media they have, and then the canister would handle the mechanical side of the spectrum.
To be honest I'm not even sure about the shovelnoses, they seem very skittish from what I learned about them and also very boring.
So it's not even possible for 20% w/c weekly. Like I said in my last response, would 30-35 be enough for just the oscar and the plec. Sorry for double posting.