I have a very strong feeling your just not changing enough water. Either that or dead fish in the tank somewhere.
Over the past 25 years I have always done weekly 50% water changes and had what Id consider to be 90-95% success rate with everything Ive tried to do, especially spawning harder species. I went this big this time to try and not have to maintain that rigorous of a schedule and quite honestly, don't feel I should have to with the stocking, plants and amount of media.
I feel like something is not right, not that my changes aren't enough. I change 50% every 10-20 days as of now...and have had smaller tanks with bigger and messier stock that didn't have 10ppm after 7 days...
Ive even had weeks prior where I couldn't do water changes on a 180 with pikes for 2-3 weeks and still never had more than 40-60ppm nitrates.
Oh, and Ive tried 2 kits so its not the kit. Thanks!