Test kits are available in strips (dip sticks), tablet/powder, and in liquid form.
Test kits are generally reliable within a margin of error of plus/minus 5%. Once you have decided on a test kit avoid constantly switching or double checking with a different test kit, since the results may greatly vary due to varying margins of error.
The one margin of error all test kits have in common is the misuse by the hobbyist. 
Before buying a test kit check the expiration date on the packaging first. This is not only to avoid false readings, but also to ensure that the kit does not expire before you are able to use it up. Generally test kits have a shelf live of 6 months to 2 years.
Packaging is no less important. Dip sticks are sensitive towards humidity and exposure to air. Dip sticks should be packed individually or in an aluminum and air-tight container such as the MultiStick. Once the dip sticks are removed for testing, contact with the test squares should be avoided. Touching them may result in false readings.
Directions on how to use any test kit should be followed to the point. Some require shaking or for you to wait a certain amount of time between the application of different solutions.
Test kits should be stored at a dry and cool place.
Water parameters change during the day i.e. pH levels are generally at their lowest in the morning hours. Avoid the confusion of naturally fluctuating levels by consistently testing at about the same time of day.
Always test the water before performing water changes.
Ammonia neutralizing agents will result in false readings using the Nessler method. Tests will come out positive, while in fact the ammonia can be 0. A Salicylate based kit should give an accurate reading.
The Nessler method has an amber color chart, while the
Salicylate has a yellow/green sometimes blue color chart.
Most nitrate test kits transform nitrates back into nitrites before measuring the combined values. This is only of importance if nitrites are present. Since nitrites are at 0 in established tanks, this “combined reading” is accurate regarding nitrates. If nitrites are present, the nitrate value is the combined measurement, resulting in an actually lower nitrate concentration as indicated.
When testing tap water, the water has to be de-chlorinated first. Just add a few drops of water conditioner. Chlorine can cause discoloration by its bleaching action, or in some cases form chemical reactions, also resulting in false readings.
Liquid test kit results can also be influenced by the appearance of the water. Any discoloration of the water, brown, green, or white, will darken or lighten the test water in the vial and result in inaccurate readings when compared to a color chart.
Most pet stores offer water analysis, either at no cost or at a small fee. Note that only a few tests can be done reliably, such as hardness, specific gravity, copper etc. Other water parameters such as pH or ammonia should be tested directly on site as the levels will change within a short period of time after the sample has been taken.
Most mistakes occur while using the liquid test kits. The test tube must contain the correct amount of water. The amount of water needed for testing must be accurate in order to avoid false readings. Since all of these test kits rely on a very small amount of water, the margin of error is quite high.
The elapsed time in-between applications of solutions and/or readings is also a common cause for bad test outputs. Be sure to watch the time closely as recommended by the individual test kit.
Powdered (tablet) form test kits harbor the same potential errors as the liquid test kits.
Insufficiently dissolved tablets, wrong water levels, incorrect timing or mixing ratios, all sum up to a combined mistake and inaccurate test result.
