Goldfish rank at the top of the list of ornamental fish species that are released by humans into the wild, as their keepers either grow tired of the fish, or have decided one way or another that they don't want to keep these fish. Without thinking, the release of goldfish into lakes, rivers, etc. all over the world can have some drastic negative impacts, not only on other fish, but potentially a vast array of other animals in that ecosystem.
In the study I am sharing with you below, a group of Spanish researchers show the potential negative impact of such an action on amphibians. Here, the study authors used the Alpine newts, and showed that by simply sharing cohabitation with goldfish, these newts hide more often, and exhibit less sexual activity.
This is the study abstract
and the full article
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0082736