Welcome to Science Buddies, I apologize for the long delay in responding to your inquiry. I hope the following information will still be helpful for you:
I think you are doing this really excellent project form the Science Buddies website:http://www.sciencebuddies.org/science-f ... p026.shtml
Blue green algae work well in a microbial fuel cell. However, I think it would be difficult to measure the microorganisms directly. In a MFC, the microorganisms actually form a biofilm on the surface of the electrode and this is necessary before voltage is produced. It is difficult to measure microorganisms that are densely packed in a biofilm.
The current produced depends on the rate of electrons moving from the bacteria as they use the anode electrode as a terminal electron receptor for energy production. So the current is a reflection of the overall metabolism of the microorganisms, and this can change with a change in temperature, oxygen conditions, sunlight or food availability. It's probably going to be difficult to maintain a high voltage at all times and get really nice reproducible graphs.
Since V = IR, (Ohm's law) you could increase the voltage if you reduced the resistance in the system. What is the concentration of ions in your tap water? What type of ions are present? It sounds like the tap water decreased the resistance in the cell. Distilled water would probably increase the resistance and so would probably decrease the voltage. It could be that adding tap water rinses out waste products that are inhibiting cell metabolism. Can you think of any other possibilities?
I think you have done a great experiment here with the tap water. I recommend repeating the tap water results to see if they are reproducible and doing more background reading to see if you can determine what the tap water is doing in your system. However, your project is due soon, so you should really concentrate on writing up the results for your display board. http://www.sciencebuddies.org/science-f ... oard.shtml