nitrate problem

Heavy fishload, dying or unhealthy plants, and high ammonia (chloramines) or nitrates in the tap water you are using for your water change. Any or all of those could cause nitrates to go up. Have you measured your tap water for nitrates.

A heavy fishload could result in more nitrate in than your plants use up. Unhealthy plants won't use up as much nutrients. High levels in your tap means you are just adding more when the 50% is supposed to "reset" the paramaters. I had that problem with my phosphates. My tap had enough, and my plants did not use it up fast enough, so I was actually increasing my PO4 with each water change and dosing.

Have you checked your ammonia and nitrite levels lately? Maybe something happened that upset your bacteria cultures?

 

Is this an EI tank? When I had an EI tank, my nitrates were 40ppm on water change day before the change and 20ppm after the water change ... just like yours. It was not an issue for me (my tank did too well in my opinion), so I'm not sure what the problem is here -- seems just about right. I did switch over to 50/50 KNO3 and K2SO4 when my fish/critter load got up there (75-80 fish/shrimp/snails in a 50G tank).

Perhaps you are skipping a KNO3 dose (or substituting K2SO4) and it is still staying 20ppm or above? In that case, one of the things that Dmax listed are probably the cause.

With my tank, when I was on vacation for 2 weeks and I had someone taking care of my tank, I skipped one of each macro/micro dosing each week since there would be no water change that week. When I got home, my nitrates were 10ppm or below ... partly also because I had a big jungle (so much so that the fish had hardly no place to swim!). So, my tank would use up the nitrates if given the opportunity.

 

I hope you have calibrated your nitrate test kit. See http://www.barrreport.com/estimative-index/3263-how-make-no3-po4-reference-solutions-repost-left-c.html for how to make the standard solutions needed to calibrate a test kit.

 

cc:

In case you do happen to get access to a decent scale (I finally went out and bought one)...

1g of KNO3 to 500ml of distilled water = 2000ppm Nitrate solution

Add 12.5ml of this solution to 487.5ml of distilled water and you will have 50ppm Nitrate calibration solution.

I actually reduced the 50ppm down to 10ppm because 10ppm was a much easier color (orange) to identify with my test kit -- all of those reds look too similar on my API kit.