Here, we add information of the idea we abandoned:
Cheap sensors and household equipment.
Before we focus on education, we started by making a household device with cheap sensors. However, after lot of researches, we found that the sensors are expensive and too technical and difficult to DIY or hack any of them.
The expensive sensors include dissolved oxygen sensor, ion sensors such as iron, nitrite, nitrate and ammonia.
https://www.sparkfun.com/products/11194
https://www.amazon.com/Dissolved-Oxygen-Meters-Accessories/b?ie=UTF8&node=5088419011
https://community.particle.io/t/sensors-for-ammonia-nitrate-nitrite/6379/11
https://www.sensorex.com/dissolved-oxygen/
Then, we tried to make a automatic device for paper test or test strip. Test paper is far more cheap.
https://www.hach.com/ammonia-nitrogen-test-strips-0-6-0-mg-l/product?id=7640211610&callback=qs
https://www.hach.com/nitrate-and-nitrite-test-strips/product?id=7640211606&callback=qs
http://www.sigmaaldrich.com/catalog/product/aldrich/z166413?lang=fr®ion=FR
http://www.mn-net.com/tabid/10327/default.aspx
https://www.coleparmer.com/p/advantec-ion-test-strips/65714
The test paper machine we designed as below:
/image later/
------------------------------------------
Automatic feeding system and Artemia
We once thought to add an "artemia farm" to grow artemia, which could be served as food for fish. Artemia used as fish food is common. We designed a mechanical arm with net for catching artemia, as below.
There were two motor to control the movement of the arm and the up and down of the net. A LED light was added to attract artemia, which is phototaxis.
-------
There was also an water level sensor to detected the water level. And for feedback, a pump should pump water from the fish tank to maintain a certain level.
However, we found it hard to grow artemia itself. We tried but they died after living for less then one week. The reason maybe the absence of food for artemia, yeast. Therefore, we abandon this idea, since it is super complicated to add artemia and yeast, a long food cycle.
------------------------------------------
Then, we focused on education, and added a LED light to represent four important parts of this system: fish, pump, vegetable and syphon.