[smokeythebandit]

I recently purchased a fish tank and I'm interested in automating certain parts of it.
This is the first time I've done this, so it's safe to assume I'm a complete noob on everything that is involved with running this kind of ecosystem.
After reading a bit, if I understand it well, this concentration of various molecules in the water is *very* important to the well-being of my future fish.

So far as I can tell, bacteria play an important role in maintaining the right chemical balance of the water.
I've purchased a some fluid which already contains the right bacteria colonies to regulate these chemicals.
The excrement that the fish produce has a high concentration of ammonia, which the bacteria will convert to nitrate which my plants can 'eat'. (Ammonia -> Nitrite -> Nitrate)
Right now I have to wait for the bacteria to grow, in about 2-4 weeks some sort of equilibrium should have formed.

However, various sources tell that this balance is very delicate. It will be disturbed by things like: refreshing the water, putting in different fish, temperature fluctuation, variations in light intensity...the list goes on.
Like I mentioned before, I want to automate my fish tank. Ideally I would like the 'system' to automatically restore the balance when it is has been disturbed.
To achieve this I think I need to monitor the concentration of these substances.

I have a background in electrical engineering and programming.
In high school I had some classes on chemistry and biology. Its been a while... but I think/hope I still know the basics. Regardless, my knowledge is basic at best.

Any advise is greatly appreciated!

[Borek]

Matter of sensors - somehow I doubt you will be able to make them. Controlling them and reading the output should be (with your background) trivial.

Not that I know what sensors are best. I would start searching for ion selective electrodes, as these are usually the simplest to operate, you just read the voltage.

[smokeythebandit]

Thank you for your response. I've done some further research. For sure I won't be able to make these sensors myself.
While searching for what sensors to use I've found a few that seem to match your description:

https://www.cooking-hacks.com/nitrate-ion-no3-sensor-probe - €142.80
https://www.cooking-hacks.com/dissolved-oxygen-sensor - €600.00
https://www.cooking-hacks.com/ph-sensor-probe-for-smart-water-ions - €37.80
https://www.cooking-hacks.com/single-junction-reference-probe - €54.60

I think these sensors are meant to be submersed in fluids.
One thing I noticed is that vast price difference, is this due to that materials that are required to produce them ?
In the article description of some of these probes it is also mentioned that I will need a 'Single Junction Reference Probe'.
Is this because the presence of some molecules will effect the concentration measurement of the molecule I'm actually trying to read?

I also noticed that there are sensors out there that can measure the concentration of molecules in the air.
These seem to be generally cheaper than those that are meant to be submerged.
Probably the ammonia, nitrite and nitrate in the water are picked up by the air. Would it be okay to measure the molecule concentration in the air, and then deduce the concentration of molecules in the fluid using that information?

[Borek]

I think these sensors are meant to be submersed in fluids.

Yes.

One thing I noticed is that vast price difference, is this due to that materials that are required to produce them ?

Probably.

In the article description of some of these probes it is also mentioned that I will need a 'Single Junction Reference Probe'.
Is this because the presence of some molecules will effect the concentration measurement of the molecule I'm actually trying to read?

No. To measure the potential difference (which is what you will be really measuring) you need a reference electrode of a known absolute potential - and it has to be connected with the solution to close the circuit. Just submerging it doesn't do the trick, you need some kind of a junction. Some electrodes (especially pH electrodes) often come with their own, built in references, but that's not always best course of action, so sometimes we use an external reference. Note: one is enough for several other electrodes.

I also noticed that there are sensors out there that can measure the concentration of molecules in the air.
These seem to be generally cheaper than those that are meant to be submerged.
Probably the ammonia, nitrite and nitrate in the water are picked up by the air. Would it be okay to measure the molecule concentration in the air, and then deduce the concentration of molecules in the fluid using that information?

No, ions won't make it into the air. Ammonia is just a dissolved gas so it will be always present above the liquid as well, that's not the case with NO₃^- and NO₂^-.

Knowing pH and concentration of ammonium ions (NH₄⁺) you cna always clauclate concentration of dissolved ammonia though.

Note: electrodes are good for continuous control of the concentration. It is probably not necessary, changes are not very fast and checking concentrations of ions once a day can be enough. That can be done with tests that call for manual action - some kind of strips of solutions that are mixed with water and change color allowing you to read the concentration by comparing with a printed scale. Can be much cheaper.

Measuring pH with an electrode (and DIY meter) can be fun though and shouldn't be expensive, pH electrodes are common and finding a reasonably priced one shouldn't be a problem.