Most plastics are safe as far as the material itself but then there are times when we find a "mold release" which is somewhat lick/slippery left on the plastic. Since I don't know where items have been stored or what might be left on them, mold release, sprayed with insecticide, etc., I just do a simple cleaning, much as I would for new dishes I might use to eat my meals. There might not be anything there but I want to KNOW they are clean?
So before any decor, etc. that I don't know about goes in the tank, I do a simple bleach soak. It doesn't have to be heavy but just a touch stronger than the normal chlorine we have in tap water. I have a laundry sink in the garage and it is easy to run a few inches of water, add a splash of bleach and let it set overnight. When I get back around to the project, I do a rinse to dilute the bleach and treat it much as I do tap water since the only difference in tap water and bleach soaking is how much water to dilute the bleach.
But when looking for plastic to plant and bury in the sub, I look at my recycle bin first as there are so many plastic items there all ready to be cut down to what I want. That way, if I cut it too much, it's okay and I just get another one out of the trash!