Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
I rinse out with tap water a couple of times.
If it's a test for something the tapwater contains, it's possible some residue may be left behind. So when testing again, I fill the vial halfway with tank water, cover, shake, and empty. Then refill to the test line and test normally. I think that should take care of it for the hobby-grade tests, as they're just not that sensitive.