structure for DIY float tank
The frame is not supporting the weight of the water, but is used to provide a form for the canvas/vinyl that contains the water. Standard galvanized piping threaded into pipe fitting would be quite strong.  You could use fittings like you linked to or standard threaded pipe fitting.  You would need to add more vertical side braces on the sides and ends than in the Zen Design to support the vinyl water bed.  Their design is made with waterproof canvas.
To provide better support for the sides of your water “pond” or bed, I would line the ends and sides of the pipe frame with plywood.  This would provide better side support for your vinyl bed. A sheet of plywood on the bottom would also provide protection to your bottom of your float tank. 
With the use of pipe fitting, no &quot;soldering&quot; would be involved.  You would have to use some pipe “union” fitting to complete some of the joints if using standard pipe connectors.  With the connector you linked to, No unions would be required, as those are slip joints, held by set-screws.
Edited to add Additional Notes: Notice that you stated &quot;tubing&quot;, and I was thinking about galvanized pipes.  Galvanized conduit used for electrical work could also be used.  The fitting for electrical conduit uses set screws which are similar to the type the Zen used in their assembly and the ones you linked to.  This would be adequate for a float tank with 10” of water.  You would NOT use this type of frame assembly to make a deep spa or a deeper float tank.  Ten inches of water would most likely be your maximum depth.