High nitrate with one test kit, not so much with another

I have multiple test kits for testing my water. Two different bottles of Tetra strips and two test tube kits. The test tube kits both show that my water is pure nitrate with almost no water. The strips both show a lower level, but do not always agree with each other. I have another brand of test strips which generally agrees with the Tetra strips. This past week I thought my nitrate spiked to a ridiculous level and did a roughly 75% change and the nitrate levels did not drop a bit.

I used to be able to keep on top of this, but now every test I do has sky high nitrates except for the 180 I got from Bob with the sump on it. That one only shows moderately high nitrates. Also, my planted tank shows sky high nitrates as well and all it has in it is a 4 inch pleco and guppies.

I'd suggest you take a water sample to a local fish store and compare their results to yours. Very likely your test reagents have expired.

Willie

2023

Did tap water test zero nitrates? Or at least < 10 ppm?
If yes, your strips and colormetric solutions maybe accurate

Next obtain a known standard solution of nitrate. Test the standard and results should reflect close to know standard value. If the reading is close, then all your testing supplies are correct. Conversely, if test of known reference solution is not read correctly, then the kits and strips are faulty and should be returned.

If the reference solution is read correctly, then are two possible causes. One is over feeding, two is inadequate water changes, or both maybe the problem.

I agree with having another party check values with their kits as they likely reference check their kits often. Companies selling water softeners may be willing to test. Ask to have tap water and tank water tested.

Tap water tests as zero nitrates. At one time, I had dangerous nitrates coming out of the tap.

They fixed it quickly as apparently high nitrates is really bad for human babies.