Electrolysis of water for demonstration: ideal electrolyte for safety, stability, conductivity, and ease of maintenance I'm developing a system for educational purposes where electrolysis of water takes place. The aim is to practically show how electrical energy can be converted to mechanical, by electrolyzing water, igniting the mixture of $\ce{H2}$ and $\ce{O2}$ produced, and accelerating a mass upwards.

Currently I'm using citric acid in distilled water ($4\%$ in weight, $18~\mathrm L$ water), platinum-coated mesh electrodes, $500~\mathrm W$ max power supply ($0\!-\!65~\mathrm{V_{DC}}$, $0\!-\!10~\mathrm A$). I'm wondering if there's any better option that meets the following requirements:


Not harmful to maintainers (that have to prepare and replace the liquid) and bystanders
Stable (not producing precipitates, and in general that can minimize the need for replacing the liquid)
Low/no interaction with the materials in the tank (Plexiglas, brass, platinum, silicone tubes, ABS)
Maximizes conductivity (the current setup at $20~^\circ \mathrm C$ has a conductance of $250\!-\!300~\mathrm{mS}$)
Not suitable for microorganism development (in the current setup algae develops in about two weeks)


If the last point can't be improved with just the electrolyte, I'd kindly ask for a suggestion for an additive that is not going to spoil the points above.
I suggest $1\%$ w/w $\ce{NaOH}$. Point-by-point:


$1\%$ w/w is $\sim\!10~\mathrm{g\over L}$, which for $\ce{NaOH}$ at $40~\mathrm{g\over mol}$ is $\sim\!0.25~\mathrm{M}$, corresponding to a $\mathrm{pH}$ of $13.4$. Standard personal protective equipment (coats, gloves, goggles) should suffice for observers and maintainers.
$\ce{NaOH}$ is extraordinarily soluble, so you will have no difficulties with precipitation.  You will want to keep the system covered when not in use, if you're not already, to minimize evaporation.  You may get some riming on the interior, but rinsing with DI water should take care of it.
According to Schweitzer$^\dagger$, the materials in your system have the following resistances to $10\%$ (not $1\%$) $\ce{NaOH}$:  


Plexiglas (acrylic): Resistant, to $80~^\circ\mathrm F$
Brass: Good (less than 0.02" penetration per year), to $200~^\circ\mathrm F$
Silicone elastomer: Resistant, to $80~^\circ\mathrm F$
ABS: Resistant, to $140~^\circ\mathrm F$
Platinum: ..... darn well better hold up! (Though, if there are any gaps in the coating, the substrate might be exposed to attack.)

Per the first chart here (PDF link), the conductivity of $1\%~\ce{NaOH}$ should be about $50~\mathrm{mS\over cm}$, which is over ten times the $\sim\! 4.5~\mathrm{mS\over cm}$ of your current $4\%$ citric acid recipe.
I salute any common microorganisms able to grow in a $\mathrm{pH}~13+$ solution. (See, e.g., Table 3-5 here, where the maximum survivable $\mathrm{pH}$ for any species listed is $11$.)


Obviously, if you should switch, monitor the health of the system materials to be sure the above literature chemical resistance data actually holds in reality.  There's always the chance some small component of the system is of a material you haven't listed, too.

(To note, I re-thought the vinegar-plus-sodium-bisulfate approach after discovering that both brass and acrylic/Plexiglas are poorly resistant to vinegar, and that the bisulfate is not likely to provide as much of a conductivity boost as I was thinking.)

$^\dagger$ P.A. Schweitzer. "Corrosion Resistance Tables, Part C, P-Z." 4th Ed. New York: Marcel Dekker, 1995, pp. 2685-2688. (Amazon link)