Copper wire works best. It bends more easily than (steel?, tin?) wire and tins (wicks up the solder) more easily without melting the insulation. The *really* nice stuff has silicone insulation, which handles high temps without melting and flexes really nicely, but it's probably overkill for tanks. A light gauge speaker wire would suffice, maybe some stranded Cat5 network cable for LEDs and such.

D.