Potassium, correct?

It might be early potassium deficiency, but it’s mild. Give the plant a few days to respond to the increased Alga Bloom. Early potassium deficiency signs -Color loss starting at the edges of the leaf (margins fade first). -Leaves turning a dull, washed-out green or grey-green. -Tips and serrations sometimes slightly yellowing before browning later. -Often shows up during early flower because the plant’s K demand jumps. -Your leaf fits several of these. But keep in mind: this can also be caused by 1. Light stress Leaves directly under intense LED light often show: -Pale topside -Slightly bleached edges -Uniform desaturation without necrotic spotting If this leaf is from the upper canopy, check your PPFD—it might be a touch high. 2. Mild magnesium deficiency Magnesium (Mg) deficiency can show: -Pale areas between veins -Overall desaturation -Sometimes misidentified as K deficiency Mg needs also increase in early flower. 3. Soil/feeding mismatch You said you're using Alga Bloom 4 ml/L. Bio·Grow/Plagron’s Alga Bloom is organic and slower-release compared to mineral nutrients. It’s not uncommon for organic feeds to lag behind the plant’s sudden K demands when flower begins. Meanwhile: -Keep pH 6.2–6.5 for proper K uptake -Make sure light isn’t too strong on that leaf -If it spreads, add a small K booster or a bit of Epsom salt for Mg support Right now it doesn’t look serious—just monitor and adjust lightly. Hope that helped! 😘