Clear vinyl tubing makes a good water fill line between your gravity fill and fresh water tank. It is very easy to great a no-leak connection with standard hose clamps.
The only limitations of vinyl tubing is that it is prone to crimping when making turns. Keep your run straight and your turns gentle.
Wall thickness is 1/8".
Available in 2 sizes: 1-1/4" ID and 1-1/2" ID.