Research
Carbon Dioxide
Carbon dioxide (including Carbonate hardness since they're the same probe) is likely something we can't measure since the useful pH range is only 4.8 - 5.2
EC (For Calculating Salinity)
Measuring EC is tricky because it's succeptible to electrical interference which is introduced by having AC pumps submerged in the water. If you decide you want to measure EC than make sure you have at least a ground probe in your tank.
Most of the circuits that I looked at either used a 555 or a complicated AC generation circuit that measured the current. (Updating)
Ammonia
The goal here is to enable measurement of Ammonia by the hobbyies. We'll be listing recommended probes as well as detailed information about how they work as well as how to configure them.
How does the ammonia ISE (Ion Selective Electrode) work?
In order to understand the limitations of ammonia measurement, it is important to gain a fundamental understanding of the how the ammonia ISE works.
Dissolved Oxygen
Dissolved Oxygen (DO) probes are availabe in two different types. The first type is polarographic and the second is galvanic. The difference between the two is that polarographic puts out a small current and galvanic puts out a small voltage. Since we've already developed circuits to measure small voltages (BNC Sensor Shield) we can use it and adapt it to DO's characteristics (namely it only produces 40mv at full saturation).