NTLabs Cristalclear 500ml

500ml treats 2000 gallons once.
When to use:

Use with the build-up of nuisance plants such as duckweed. Cristalclear removes phosphate from the water, the nutrient responsible for the rapid spread of duckweed. Add to the pond every week for the first 2 weeks and every other week thereafter during the growing season.

Do:

Shake well before use.
For best results, rake off as much duckweed first.
Mix full dose in a clean bucket of pond water, and then pour evenly around the pond.
If the water is very badly affected the dose can safely be doubled without risk to fish.
Use for a minimum of 6 months to achieve the optimum benefits.

To calculate pond volume -
length (ft) x width (ft) x Depth (ft) x 6.25 = pond volume in gallons.