GE silicone 1. Like others said, don't get the GE 2 because it has the mildew protection. You should be able to find the clear ones in most hardware store. Where I live, I can only find the black GE 1 silicone at ace hardware.
black silicone does make a nice looking tank make sure and run some painters tape( the blue kind) on both sides of you seam however wide you want it to be smooth the seam with your finger and remove tape immediately makes a nice clean seam