If everything is healthy as you report, don't worry too much about your nitrates at this point. I also run a Rena canister on my current set up but as per the previous responses, I've removed all media except for a bag of carbon. If you run your canister that way (empty baskets), you can almost consider it as a sort of closed loop or mini sump. My system was also acquired used and after the initial cycle I had slightly elevated nitrates, then they dropped to zero only to come back up to just under 20ppm. Wasn't until I had been running the tank for close to two years that the nitrates dropped to zero. I did add some Chaeto macro algae courtesy of Myka about a year ago and soon after that the nitrates dropped. Not sure whether it was solely the addition of the Chaeto or the tank & live rock maturing sufficiently. I also added significant flow in the form of a VorTech propeller pump a year ago. Could have been a combo of all those things that eventually took care of the nitrates. I have a similar fish load with two maroons (just deposited their second batch of eggs) and assorted other species.
Curious about you running the canister off the 'overflow'. Doesn't the canister have it's own intake and output? Why would you run it from an overflow and what kind of overflow is it?
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