Single Mosquito Dunk (BTI pucks). Often used indoors in potted plants for fungus gnat control, targeting the larvae of this common pest (although this is not a registered use for Mosquito Dunks in Canada, so there are no specific application instructions included with the product; most home gardeners crumple 1/4 of a Mosquito Dunk into a 6-8" pot and water to activate). 

For a more detailed discussion on managing fungus gnats, check out our blog post on the topic.