Phosphate (PO4) is present our water sources and therefore aquarium.
While phosphate does not directly harm your fish, even at high levels, the algae blooms that result from elevated phosphate can ultimately cause problems for the aquarium inhabitants, however.

Green water that is high in algae can change the water pH (acid-base balance) and decrease the oxygen, which can harm the fish and even result in death.

Phosphate naturally increases as wastes are broken down within the aquarium. In addition to being internally produced, phosphate can enter the aquarium from external sources. Everything from fish food to the chemicals used to buffer the water to the tap water itself can contain significant amounts of phosphate.

Phosphate sources may include uneaten food, plant decay, fish waste, carbon filter media, aquarium salts, pH/kH buffers and even our tap water.

Phosphate can be reduced by,

• The primary source of phosphate in an aquarium is flake food. Cut back on the frequency and amount of food and remove any uneaten food.
  
  
• Regular water changes will help keep phosphate levels from rising. Change 10 to 15 fifteen percent weekly, using a low phosphate water source.

• Keeping the tank free of debris and algae will help avoid phosphate build up. Vacuum the bottom frequently to remove uneaten food, plant decay, and fish waste.
  Cleaning the filter media helps as well.

Instructions:

• Add 10ml of aquarium water in to the test vial.
• Add 4 droplets of the PO4-1 reagent and swirl gently for 10 seconds.
• Add 1 scoop of the PO4-2 reagent and swirl gently for another 30 seconds.
• Place the test vial against the color chart to obtain reading.

Planted tanks usually require a phosphate level of 0.25 – 1 mg/L (=ppm) 

Note:
Keep out of the reach of children! Only for aquarium hobby use.
The use of protective gloves and eye protection is recommended.
Please read the instructions carefully before using the product. 

Salifert:
Salifert is a Netherlands base company that produces top of the line test kits as well as tank additives and foods for marine and reef aquariums.
Salifert is well established and popular with hobbyist for their accurate and affordable test kits.