First, we don't recommend keeping fish with turtles, typically, for a lot of reasons. One is most turtles are kept in tanks that are too small anyway, so water quality is not very good. Another is that turtles will generally eat fish. Your pleco isn't very visible to the turtle because of his coloration, and the fact he doesn't move much so he is relatively safe from the turtle.
In regards to the Ick Remedy though, no, it is not safe for turtles. Turtles will absorb chemicals through their skin. The only one ever recommended for a turtle tank is a water conditioner or de-chlorinator.
Date published: 2017-05-11