The FET has been developed to replace acute toxicity tests on adult fishs. A report commissioned by ECHA provides background information about the FET test.
Read more here:
https://echa.europa.eu/documents/10162/21650280/oecd_test_guidelines_aquatic_en.pdf/2548af92-ffe1-4e38-a42a-463103b1586f
OECD and EU test guidelines:
https://echa.europa.eu/support/oecd-eu-test-guidelines
Current FET report of ECHA and UFZ (Environmental Research Centre Leipzig)
https://echa.europa.eu/documents/10162/13639/fet_report_en.pdf/b6036bdb-9041-41c8-a390-d9b66b244a4b