Honestly I do a 20% water change per month
Not for nothing, but I change 55g every 3-4 months in my 240g. If it ain't broke, don't fix it. I do the WC for the Trace Elements, other than that, most dose something or another in the tank. Nitrates never go above 20. If your Water Parameters are fine, why mess with them. Unless you are not dosing anything, then you might want to up that some.
My system is around 250 gal and i am doing a 23 gal water change weekly which comes out to 40 to 45% monthly and since I use around 5 to 8 gals a day in top off water which I firmly believe is changing water I think my water is getting over 100% water change per month I know some dont consider top off water a water change but using say 5 gal a day for top off that in my opinion IS a water change maybe not the elements you need but it is fresh water ,,,
You may be adding FW to your system, but evaporating water does not take Nitrates and other things with it. On the other hand, if they don't get high enough to warrant an export than its not necessary.
,,As I'm doing the ZeoVit program and it's a daily based dosage of trace elements among other stuff I think I don't have too much trouble, though, I wouldn't advice on not doing a water change for a big tank at least a month.