Bad Fungus?

did your grow box are made from melamine board ??? if yes switch your plant between to box, the sadly plant should recover in 1 week, in the same time the healty one should start to show some trouble. the melamine board start to degrade in time with heat, uv, and humidity.... they start releasing C.O.V. many, many years ago i loose some 4 years old mother plan this way, your photo really remember me how they slowly die...

k-deficiency, though pic 3 and 4 is something else. interveinal chlorosis is caused by more than 1 thing. K deficiency does have this, but usually you see damage on the serated tips before or at minimum coinciding with it.

Severe potassium deficiency, not a fungus issue at all. Last picture looks like burning as well, maybe due to constant under watering/not watering to run off each time, causing a build up of fertilizer salts in the substrate........but without a diary and knowing exactly what you may or may not be doing.........

I agree with K deficiency and maybe some Mg too. Fungus that cause trouble on living plants is either a mildew or budrot. The mildew looks like white powder and budrot is grey or brown and will kill the area it is growing on. You don't have those, so it's definitely a nutrient issue.

Looks more like an K deficiency to me. With fungus you have excess and deficiency at the same time and it most often starts with calcium in my experience, and you notice that the plant stops drinking. Also the leafs don’t look fungus droopy here, but the shoot on the 5th photo did you bend this one or did it happen on its own? If you are still concerned of a fungus you could add some H2o2 (12% 0.75-1ml per liter of water) or try some beneficials (don’t mix with H2o2). But id say they look too bright to me and leafs not hanging i would recommend to feed her. Maybe you could give us some more information about your feeding schedule, EC and PH if you already tried giving her nutrients, what kind of lights are you using, do you give extra calmag?