[yak00za]

I am working on my project researching water samples from different places.
I was going through identifying some ions in there and have successfully found Cl^- in most samples.

Then, for unknown reason I decided to add FeSO₄ to all samples and see what turns out of it. The results were interesting.
All samples except for distilled water became very-very light yellow, and then colours started saturating. After nearly 30 seconds they have reached their final shades that can be seen on the image below:
https://imgur.com/a/qN3Ktcq

To add a bit of clarity:
Sample 1 is drinking water from the water cooler in the school building.
Sample 2 is tap water from one of the city apartments.
Sample 3 is well water.
Sample 4 is river water.
Sample 5 is distilled water.

I decided to investigate this and try to identify what exactly caused this reaction.
Because the colouring process was relatively slow I thought that Fe²⁺ were just oxidising to Fe³⁺ ions and that's why the colour was becoming slightly yellow and than darker over time.
Then I tried heating the reaction product. In the result, colour has changed from yellow to brown, and particles of dark precipitate became visible as shown on this picture: https://imgur.com/a/CdxE6vL

That also looked like a Fe³⁺ for me, so I tried checking it with NaOH. Dark green swamp-like precipitate in the result. The ions are still Fe²⁺.

The thing is, most of my ideas are cut off by other reactions I tried.
Addition of NaOH, HCl, H₂SO₄, Na₂SO₄, Ba(NO₃)₂ resulted in nothing.
As mentioned before, addition of AgNO₃ showed Cl^- ions in samples 1, 3 and 4.

What are the variants of the ion that causes this yellow colour in there? I will hang out in the lab soon and I want to know what to look for, because right now I have literally no idea of what it can be. Other reactions that show nothing are confusing me.

Thank you for your time!