When my 90g was a true
FO with a wet/dry, I only performed a 25 to 30% water change every 3.5 to 4.5 months. This went on for about 10 yrs. Since it was a
FO tank, I've had my ups & downs with specimens but the levels were never so out of whack that I had to perform a water change. I did not see any significant down side to the lack of water changes, although at that time I did add weekly trace element additives. My
NO3 levels were always on the low side for a
FO tank. (10-15
ppm) I attribute this to the plenum that I placed under the
CC about 15 yrs ago. Interesting that the
NO3 levels stayed high for the first 4-5 yrs. (80-100
ppm)
The tank has since been converted to a semi-
FOWLR, (kept the wet/dry running) and since I waited 15 yrs to purchase a
RO-
DI unit, I've changed my water changes to about 12-15% bi-weekly. I'm sure that it's just a new toy thing (
RO unit), in a couple of months, I'll be back to monthly, then quarterly.