Also regarding testing and rinsing. Clean the test tubes and cap both after testing and then the next time before testing. After rinsing in tap water right before testing, rinse with tank water as well to get any chlorine/chloramine out. And don't forget to do the caps with the vials.
When testing it is best not to take the water from near the surface where oils and other things tend to collect as they can throw off test results. Get the water from more like mid-level. If you clean your vials properly, you should be able to put them, open end down, into the tank about half way down and then invert them to let in the water. Then pour out enough to get it to the line. A good trick here is to fill the cap so if you take out a bit too much you can add back from the cap. Alternatively, you can use one vial as the designated scooper and never worry about accidentally getting something nasty into the tank. Scoop with the designated vial and pour from there into the actual test vial.
Never use your bare finger to cover the vial when mixing the reagents. Be sure to use the cap.