Yes it is safe to use but not a very high tensile strength. I used asi aquarium silicone and I believe it is rated up to a 700 gallon aquarium. I built a 4'×4'×16" frag tank with it and no problems.
Ive read its safe too. I use it to seal all my threaded plumbing connections instead of teflon tape as well as anything else that needs a glob of sealant.