For a regular monitoring of your pond water JBL offers a series of test kits. With most of the tests, a reagent is added to change the colour of the water. JBL tests have a special feature to make the reading easier and extremely accurate: Instead of only comparing the colour change of the water on a colour scale, a second measuring vial with the pond water is also put on the colour chart. This water then appears to have (optically) changed its colour, which makes it easier to compare it very accurately with the water colour of the other measuring glass vial. This system of comparison is called comparator system. It compensates natural water discoloration.