Long story short: The water tank puzzle isn't solvable on demo; it isn't playable on steam for mac (yes you can download it but you can't open it unless you use external hardware or force open which doesn't always let you actually play the game); it does cost money to play the full game on Steam (TRACE); this is the second part of the game (after purchasing TRACE it becomes a free update)
Any other questions I can answer if its about the demo.
I have no affiliates with this game I just did my research and I reply quickly :)