no. no. and no. duct tape will stay glued under water for a while, but will start loosing its adhesion. when it does the chemicals in the tape will leech into the water. there are some good uses for duct tape but aquariums are not one of them.
when you get the silicone get GE type I or 100% pure RTV silicone. nothing with any addatives or agents in it...they will leech into the water. the coke bottles themselves should be ok. MOST plumbing or other products for "potable" or drinking water are safe for fish...not all, but most.
before you dump something in your fish tank, figure out if you stuck the article in a pail of water for a few weeks if you could safely drink the water afterwards...would you drink water that had paints, or tapes, or fiberglass resin, or that rotten log in the back yard, or some unknown rock sitting in that bucket for even a week? be risky to say the least.