Closed system + heavy feeding = need for water changes. In nature, you're dealing with huge, unthinkable volumes of water, and most fish eat rarely, especially predators.
Having said that, I make 20 per cent water changes every three days on my 75, BUT i suck up the detrius my midas makes each and every day, then replace water (1/2 gallon) or whatever.
i used to do alot of water changes on all my tanks( 50 percent every 3 days) but now i only do that on an overstocked tank. just get some heavy duty filtration and 50 percent a week with some twice weekly changes will keep your tanks nice. and lower the feeding if its not all necessary, what kind of fish do you have?
I only do 20% or so every other week, but I only have one fish and its a neat eater. Filter turnover is 12x with a very strong current in most of the tank to keep solid waste moving and getting sucked up by the filters.
With the pleco, you might benefit from a 5 gallon gravel vac mid-week. I have to do this for my pleco, though sometimes it turns into 10 or 15 gallons, depending on just how much he's been eating. Also, he's in a 180, so he tends to move around and poo, and so it's hard to get all of it in just 5 gallons.
This is just my understanding. I do waterchanges so that I have the lowest nitrate levels in my tank and at the same time I can bring fresh water and replace depleted minerals in the tank. With waterchanges I believe growth inducing hormones are reduced in the tank as well.