Don't worry Oz. people used to maintaining 1000s g of commercial tanks by massive daily water changes simply can not grasp the concept of a stable bioload being balanced out and stabilized by plant life and nothing else.
But you can because you left the tank alone for months.
And Cal can because he hasn't had to clean his glass for over a year.
Meanwhile, I have come home to at least one 10g for the last 28 years with a stable population of 20-30 fish, no algae, no filtration, no circulation, and no water changes.
But then this may simply be a difference between commercial facility and tanks in living rooms and bedrooms where people have to live (and sleep). And being maintained by people who have other jobs to go to also.
So why are your nitrates up? Simple. Because the nitrate consumers have not kept up with the nitrates. So you have two options. Massive invasive, upsetting, daily water changes. Or simply add more live plants to get the tank back in balance. I hope you chose the latter
FWIW here is a nitrate test of my 10g with 20-30 descendants of the original cycle trio added over 4 years ago.
oh yea. Testing right after feeding should not vary the test results much if any at all
I hope you keep us posted. Monthly would be fine.