The API® REEF MASTER TEST KIT test 4 key parameters – calcium, carbonate hardness, phosphate, and nitrate – to keep reef inhabitants healthy. Testing is fast, easy, and accurate, yielding precise results, and comes with 4 glass test tubes with snap-tight caps and a convenient holding tub for easy, reusable storage. This kit also includes a complete, step-by-step instructional booklet with a color chart that provides information on how to correct unsafe water conditions.