Growstones Gnat Nix! is available in 9 litre and 42 litre bags.

From The Manufacturer:

Gnat Nix! The Only Fungus Gnat Control With NO Harmful Pesticides Or Chemicals

Growstone Gnat Nix! is the only scientifically-proven, chemical-free fungus gnat control on the market. It’s not just free of harmful pesticides and chemicals, it’s made from 100% recycled glass. Plants love it. Gnats don’t.

Gnat Nix! is:

• Non-toxic, chemical-free fungus gnat control
• Long-lasting top dressing
• Effective under dry or moist conditions
• Great indoors – outdoors

Scientifically Proven

University trials have proved Gnat Nix! used as a top dressing is an effective physical barrier against fungus gnats. Unlike other top dresses that are limited to one phase of control, Gnat Nix! controls gnats at different points of their lifecycle. It prevents adult emergence from hatching larvae and deters females from laying eggs in the growing media. As a result, the lifecycle of fungus gnats is interrupted, and its population significantly reduced. 

Growstones talk to the hydro store video: