On the long term, i'd like to monitor the water quality. The main parameters to monitor are dGH, dKH, pH, dissolved oxygen, CO2 in water, nitrites (NO2-) and nitrates (NO3-). Ideally, we could also measure many parameters (Amonia ->NH3, Amonium ->NH4+ )  that are part of the nitrogen cycle. However, unless the aquarium is not cycled, there should be no ammonia or ammonium. peaks of nitrites may occur when there is a problem in the tank (death of a big animal whose body is left in the tank), but shoudn't normally occur.

pH should be kept at certain levels according to the population of the tank. Some fish like high pH, neutral or low pH. In my case, i aim for a neutral pH of 7.

Another set of parameters to monitor is water hardness. 

Two indicators exist : general hardness (GH) and carbonate hardness (KH). It is often measured in degrees (dGH and dKH) with most tests. Again, here, the target values should be set according to your population. In my case, i aim for 7dKH and a similar value of dGH, since i have caridina cf. cantonensis shrimps, and neocaridina palmata in the same tank (i know the gh and kh values doesn't match with the provided links, but on other websites i found this value. It is really annoying to find the exact value...).

Dissolved oxygen is used to check if there is enough oxygen in the tank. To increase oxygen in the water, you have to make the water surface move. For that, i set the filter outlet in a way that makes movement on the surface, and i have an air pump, making small bubbles, also moving the surface. The hotter the water, the lower the dissolved oxygen in a similar setting. It should not be a problem for my setups anyway.

At last, Carbone dioxyde in water is important for some plants. Some need carbon to be added in water (i have a liquid containing it - easylife carbo -, but gaz form can be dissolved in water too). Too little and some plants don"t grow, too much and animals in the tank die. I tend to put less liquid in my tank than the specification. That way it may not be enough for the plants to grow fast, but it shouldn't kill my tank inhabitants.

Now, on measuring these parameters. The most frequent way of measuring those is chemical tests. Either strips of paper (i have been told by everyone that those are not reliable or precise), or a liquid solution. In this case, you take 5ml of water in the aquarium to be tested, and put a few drops of a solution in the water, and depending on the color, you know the value of the parameter.

This method is cheap, but can't easily be automated. One way to automate it would be to have a peristaltic pump preparing the solution, and either a color measuring sensor, or a camera to assess the color of the solution.  However it may prove difficult to fully automate, and you would still have to wash the container after mixing the solution in it, otherwise the next measure will be poluted by the previous one.

I may still work on a semi automated method, where i would make the solution by hand, and have a color sensor or a camera compare the obtained color with the reference colors. This way, it may be possible to extrapolate intermediate values, and the system would store the values rather than have to type it in for later references.

Another solution is to use probes. Probes get immersed in the water, and either return an tension or a value depending on the measured parameter. For automation purposes, it is perfect, since it is what we are used to do with temperature, humidity, etc sensors. However, the downside is that those sensors are expensive (often 100$+ for each sensor) and require a measurement board with amplification to have reliable measurements (often 150$+).

Here is an example of this kind of sensors : http://www.libelium.com/smart-water-sensors-monitor-water-quality-leakages-wastes-in-rivers-lakes-sea/