Brown spots showing up in tank

[Darth_Prada]

Hi , I would consider myself in the middle of Beginner and Advanced in this hobby but somethings have been happening that are puzzling me as I do my bi weekly quadrant cleanings of the the tank and test the water regularly with test tube kit for all normal perametters and cycled this tank first a betta fish perished inexplicably and now one black tetra that of a school of 6 has randomly perished and another one I have noticed is acting strange almost as if the organ that controls
It’s boyancy is malfunctioning but then all of a sudden it’a acting normal again but I am watching the remaining 5 fish closely for any other odd signs of distress. However unlike before when I lived in the city the water here is very hard with Iron so I have taken to using filtered un chlorinated water which I still add water conditioner too just to be safe. The only
Other thing that seems off is this brown almost
Mold lookjng film has started forming on anything that doesn’t move ( see pics) is this something i should be worried about my gut says it just biofilm
From the cycle booster bacteria I add from time to time after a water change. Can anyone confirm or have any ideas why a black tetra would act in this manner ?

Thank you for taking the time to read this any insight would be appreciated

Matt

[Caliban07]

Those are diatoms and will disappear on their own.