I have owned several hot tubs and swimming pools and am a water enthusiast.

I am in the process of designing a "mineral dip" soaking tub for my skin care spa. I am an aesthetician and own a business called Refresh where I do facials and body treatments.

I have a very old 400 gallon wood tub that I need to heat. The tub is plumbed with a flow through water system with hot and cold water coming in from a tap and a main drain and overflow drain.
I plan on using lots of mineral salts, herbs and essential oils in this tub. It will be for individual personal use but I don't want to drain it every time I use it. I will be adding new hot water that has been treated with the salt, oils and herbs that comes through a water conditioning station that I have designed. The over flow drain will take away the extra water.

My problem is I cannot find a suitable heater for my project. I do not want to use a pump for circulating the water. I am not going to have jets - I want it to be very tranquil. I want to have the heater to create thermosiphon circulation by allowing the heated water inside the heater to rise up and flow into the tub. The cold water will enter at the lower of the heater.

I also need a spa heater with titanium or stainless steel heating element to withstand the salt chemistry. I have natural gas at the site.

I realize this is very unusual and my requests are out of the ordinary but this project is a prototype for a larger spa I plan to open.

Can anyone help me design an efficient and economical heating system for my mineral dip?

Something like this?
No idea what the pipes are made of...

Copper