Large water changes input

[ocean diver]

The high nitrates could be caused by the media in your canister filter, if you don't need the canister filter I would remove it or at least clean it out once a week.

Way back when I started I did remove 80% of the water and then filled it half way up then back down leaving about 20% then filling up again, but with in a week it was right back where it was.......so I would find the cause and try to reduce that source.