Hi sinful_electron,
electrons haven't been sinful until bad guys decided to make them work that way.
What do you want to indicate in conjunction with an aquarium?
I used to have 5,000l aquariums with no indication other than temperature, shown on a temperature scale glued to the front of them.
CO2 saturation (furtilization) doesn't have to be indicated - plants do it better!
Oxygen content of the water won't have to be indicated as well.
Activating illumination (warm white and UV-A) at six o'clock in the morning you should see oxygen bubbles rising from the plant's leaves towards the surface at high noon. (If there are none your investment should go to more light sources)
PH grade of the water should be adapted to the fish's needs. (Never have breck water fish in the same aquarium as "sweet water" fish.) (PH to be checked with a PH-meter)
Last not least - clear water and happy fishes.
Keeping fish in an aquarium is almost as keeping a dog. Always have an eye on it (them)
What else do you want to indicate?
Regards
Boncuk