Yes and no. Plastic has been used for a lot of applications, food related and aquarium also, and mostly it isn't lethal. But plastic isn't completely safe. Plastics give off compounds that produce hormone interferance in humans and plastics manufacturing towns populations have experienced extremely skewed male/female birth ratios with nearly all female births.
I don't know what effects plastic offgassing products have on fish, but if it is anything like the above you wouldn't want to put the stuff in to fresh. So probably safe in the short run, but I wouldn't get to crazy with it.