Athosanswered grow question 7 years ago First, you might not have noticed it, but along with those spots, there are many leaves with burnt tips and / or edges, which a classic sign of nutrient burn, either because you are feeding too strong a solution or too often.
You don't mention anything about EC/PPM, so you are not measuring it? It's true that Biobizz is low on Calcium and magnesium, mostly because most people get it from their tap water, but if your water source is on the soft side (mine is) you need a Calcium / magnesium additive. Those purple petioles (the stalk that attach leaves to the stem) are a classic sign of low magnesium. The only way to be sure is to measure your tap water EC/PPM levels.
If you don't have an EC/PPM meter, my guess would be on the flush with Epsom salts you used. Those salts are very strong without a proper way to measure it, you most likely used too much.
Also, with a 25 liter pot full of soil , if it's a pre fertilized mix (like biobizz allmix) it's probably too early to give so much nutrients, since the soil still has enough to feed the plant on it's own.