Electrolysis of water for demonstration: ideal electrolyte for safety, stability, conductivity, and ease of maintenance
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)