Det skulle kunna vara det greger kallar sjöäpplen ,en slags sjögurka eller mask (eng Medusa) ,de kan avge ett potent gift holothurin tror jag det heter som dödat fiskarna, finns en artikel om detta om du söker på advancedaquarists register ( sudden fish death). Kolla levande stenar mm om du kan ha några andra kryp som gömmer sig. Själv har jag efter månader kommit på att jag har flera små krabbor som kommit in med stenarna och ur en sten i mitt nya akvarium kröp det ur en jätte sjöstjärna (se min blogg) ur ett minimalt hål, man vet aldrig vad som döljer sig mvh PeterG http://www.advancedaquarist.com/2002/11/inverts>>toxins of some of these species are so effective that South Pacific island cultures use the macerated bodies of these sea cucumbers to stun and capture fishes, crabs and lobsters as a method of traditional fishing (Frey 1951; Ruppert and Barnes 1994)! Despite the fact that the medusa worms lack these structures and are considered ‘relatively non-toxic’ by comparison to many of these species, this does not mean that they are by any means non-toxic. For example, the beautiful sea apples (e.g., Pseudocolochirus violaceus and P. tricolor ) also lack these Cuverian tubules, but along with Bohadachia argus and Actinopyga agassizii, are considered among the most toxic sea cucumbers in the world. In fact, according to Wilkens (1998), it takes only about 1g of tissue from any of these particularly toxic species to poison the fish in a 25g tank. These toxins also affect humans, and many people suffer moderate to severe skin and eye irritation if they come into contact with these toxins (Cunningham and Goetz 1996