I have a number of fish breeding aquariums 3' x 3' x 18" which I plan to use as Bluetongue lizard vivs.
I am planning to provide heat using a 6m 50w Heat Chord running across 3 of these tanks on a stand level. In each tank the chord will be looped around and siliconed between two ceramic tiles. The tiles will be set on bricks to provide a basking shelf within 6" of the UV light. The area under the tile will provide a hide where it is a little above room temperature and the tile's temperature will be controlled.
My question is - will the heat chord need to be insulated where it touches the glass as I loop it across the top from one tank to the next to prevent the glass cracking?

I wouldnt think mate no, but if you really wanted use could use some clear flex tubing from bunnings, about the diameter of a 5 cent peice and run the cord through that.
The only bug you might have is getting enough cord between each set of tiles to heat it much, with only 6 meters. It sounds alot but with only a few strands you really dont get alot of heat, the tiles will spread the heat aswell, so you need to account for that.

Might be easier to use the cord directly under the glass, if its 5mm glass it wont crack. you can test the floor temp above the cord and just dump extra substrate on top if nessesary. Bark chips maby, blueys love to burrow as you know, that way they can get all the heat they need.
maybe a few layers of newspaper under the substrate?

Another way is to grab some plastic plumbing conduit (use the plumbing conduit rather than the electrical, as it's rated for higher temperatures) and some curved adapters from bunnings or similar, cut two lengths a little shorter than the width of the tank, with another length a couple of inches less than 3' long for an upright. If you put the two short pieces at one end of the tank, you can loop the heat cord down the upright, through the two sections, and back up the upright piece, then into the other tank and repeat the process. The conduit insulates the heat cord nicely while retaining the heat, stops the animals trashing the cord while digging (we use conduit-insulated heat cord with big monitors, who are far more destructive than blueys ), and supplies a reasonable sized basking area without the need for excessive amounts of substrate to protect the animals (which they usually end up digging up anyway!), while still leaving enough of the cage unheated to provide a good thermal gradient for the animals. Once the cord has been fed through the sections of conduit for both tanks, and adjusted to suit, you can join the pieces together. I'd use silicon rather than the pipe adhesive, due to fumes & toxicity levels. (Remember not to put the animals in for long enough for the fumes to disappear). The 6 meters you have should do both tanks nicely, and leave enough excess to keep the plug safely away from the interior of the tanks.

What you should end up with is an upright pole, with an oval frame sticking out at 90 degrees at the bottom. This is identical for both tanks.

Aye but 6 meters of cord will stretch thin over 3x 3ft tanks i think , i used to use a 6 meter coiled u beneath one corner of a tank, most of the 6 meters and dint get too hot, but then I had a sand substraeabove.
put up up some pics when ya finish doing them eh?

Yep, it's not even worth looking at doing it my way with 3 tanks to cover with 6 meters. I used 9 meters to do 2 basking areas in 2 3' fibreglass encs.

damn good idea! have muchos problemos with boiling hot cords - even WITH a thermostat.