Bei der Pflanze sterben die Blätter

pH value: 6.2 EC value: 1.5 Temperature: 20ºC Humidity: 68% Pest control: nematodes and predatory mites PPFD: 490 µmol/m²/s At these figures (temperature, humidity, light), your plants won't take high EC... have you ever measured what's in the drain? Could be a lockout - though my initial idea is rather damaged roots due to bugs. Since you already feed mites and nematodes, my first attempt would be to restore the roots/root zone (low EC, mykos, bacteria, kelp). Do you have any chance to lower the humidity... or at least raise the temperature a little, to avoid upcoming problems (fungal infestation, mildew, bud rot)? Good luck 🙏

Advanced phosphorous deficiency. "Sand", I presume as a "gnat barrier", is not a contributing factor. It is not septoria. It is not "nutrient burning". It is not calcium related. It is not "waterlogging".

Sand in a growing medium or soil can contribute to the formation of non-permeable or oxygen-deficient layers, particularly when fine sand particles mix with other materials, become compacted, or trap water. While coarse sand is often used to increase aeration, the wrong type of sand or poor mixing can have the opposite effect. It is not sand itself, but fine sand particles (

the 2nd week of flowering is interesting, where these brown spots are forming in the second to last picture, which could be septoria, a fungus, that overwintered with you the little one on the left is already getting it too

ph lock out, and the plant is poisoned with fertilizers.

It probably started around week 5 Your plants need nitrogen for the stretch. So the first 3 weeks of flower, I still feed my plants grow and bloom formulas and then at week 4 and 5 you have to give your plants calcium and magnesium at this time. Your plants are about to swell and build buds. Calcium is the plant cell wall building block and if it doesn't have access to this at this time, it will steal it from the leaves. Both of these things, the nitrogen and the cal mag at these times will help keep the plants happy and healthy from beginning to end of flowering. You don't want nitrogen deficiency early in flower and you don't want leaf necrosis from cal mag deficiency at the end of flower. Remember this for next time.

That looks like nutrient burn to me. Do you have slow-release fertilizer in the soil? Spontaneously, I would say it's nitrogen burn. Of course, it could be something completely different, but I would consider flushing. Maybe someone else has a better idea. Have you measured the drain pH and EC? You could check what comes out/how the buffering capacity of the soil is. I'm definitely keeping my fingers crossed for you.

Looks like nitrogen deficiency

If the pH and EC are fine, then I also suspect root damage. Maybe the roots were already weakened relatively early due to the fungus gnat infestation, leading to an infection of the root tissue. As a result, the nutrients were absorbed poorly. The plant had its problems quite early on. Then you gave it more nutrients, which may have further weakened it due to over-fertilization. So I suspect you won't be able to save much, sorry.

If you checked your nutes ph and ec are good, could be problem with the roots. Rootrotf f.e.

looks like advanced N-deficiency. Looks like a Ca-deficiency started back in week 5. whether or not it's a physical deficiency or lockout requires more information. They either need more or something is too high and impeding other nutrient molecules (imbalance).

Sand in a nutshell. fine sand or deep sand layers—can cause hypoxia or anoxia (no oxygen) in substrates by restricting gas exchange and water flow. While sand is often thought of as airy, in aquatic or heavily saturated environments, its small particle size can compact and seal off lower layers, preventing oxygen from penetrating. Plastic pots, don't help but sand will just destroy oxy. As microorganisms progress through flower they scavenge more oxygen in competition with the plant. As micros slowly add more and more co2 into medium this slowly increases moisture retention of medium, once moisture in the medium goes above a setpoint, oxygen movement speed swings from 10,000 times slower than air in soil to 320,000 times slower in saturated soil. Soon as that happens its all lactic acid and 10% of the energetic output, irrespective of any other metric. Everything begins to fail.