The Glostrup City Court says Lisbeth Koelster poured 0.3 fluid ounces (10 milliliter) of a shampoo containing an anti-dandruff substance into a fish tank for a TV program in 2004.
The show on public service broadcaster DR was aimed to expose the toxicity of chemicals used in some hair products.
Three days later, 12 of the 13 guppies in the tank had died.
A veterinarian reported her to the police two days after the show aired, but the case apparently didn't get any priority treatment and was only brought to court last week.
Tuesday's ruling said Koelster knew the fish would die because the experiment was based on a similar laboratory test. The court didn't fine her, however, saying her rights to a speedy trial had been violated.
Oh, come on, they're fish. Are they going to start fining school kids for doing experiments with microscopic bacteria because they knew they'd kill them? Or how about fining housewives for killing spiders and ants.