Is it possible to drown a mermaid?

Mermaids could be drowned maliciously. Drowning is just suffocation in water. In stories, mermaids breathe air like us, or water like a fish. Fish gills take dissolved oxygen from water, so if there is no oxygen in the water, the fish suffocate. It seems likely then that putting a mermaid in a sealed tank with no vapour exchange to oxygenate the water would cause the mermaid to use up all the available oxygen in the water. She would then suffocate, or drown.