I ran a thread about the tankless heater. Yes, my darkroom tap would not draw enough flow to activate it.
I subsequently bought a 2.5 gallon electric heater. Seemed simple enough to install, sort of like hooking up garden hoses. It leaked at every junction until a plumber was brought in. My cold water tap puts out 50f in the winter, and the electric heater can't maintain constant temp for wash times. I suppose it will be more consistent as the weather gets a little better.
In retrospect, I would get a 5 gallon tank.