system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how come if I have a watercooling loop in a computer, it will grow algea even though there's no food or light. What do they survive on?
gpt
Heat and micronutrients in the water. Also there is likely some light getting in too (or at least off of LEDs in your computer, it doesn't take much). 

Think of algae growth in covered swimming pools and water coolers.

I would recommend using distilled water or a water and glycol mixture to prevent biological growth. This is commonly used in residential and industrial heating and cooling systems. 

Have a great day!