What is this brown intrusion upon my plant? And how do I obliterate it?

That is helpful @Prilyfe13. I think we can definitely narrow it down to two things - a lack of Calcium (which matches your description of brown dots progressively getting worse and becoming more blocky), and possible light burn. If the issue is only affecting the very top of the plant nearest the light, then I believe you may have hit the nail on the head with light burn. Looking at your photo dated 19th March, the issue wasn't present in it's current form then, but I can see the sides of the leaves starting to curl up. This is THE classic light burn symptom. It begins on the sides of the leaves (rather than the tips, like nutrient burn) and then progresses inwards. But, you've obviously described a classic calcium deficiency as well, as I said... so perhaps it's six of one and half-dozen of the other! When it comes to living soil, I am certainly no expert, but my understanding is that you don't generally need to add too much in the way of nutrients as this is mostly in the soil already. Hopefully someone with more experience than me in this field will be able to help you... That said, if you don't get any more expert advice, I would probably err on the side of caution and raise/dim the lights (as you have done) and then maybe incorporate some Cal-Mag into the next few feeds, just to be sure it's getting enough. I don't think I've ever seen calcium or magnesium toxicity myself, so if you're using the small recommended doses, I can't see that it would cause any harm even if it doesn't need it. RE: Fish shit - I don't think this is the cause. So long as you're only using the recommended dosages, it should be fine. RE: an equipment list - don't even get us started or it will never end lol. All you technically need is air, water, soil and the sun... that doesn't explain why my spare room looks like the inside of Area 51's weapons testing facility. 😂 PH and TDS/EC meters are great, but I managed to grow successfully for years without them - particularly growing in soil which is much more forgiving. Its the same with basically every growing accessory I own. The main thing, to me, is managing the growing environment: keeping humidity high in veg and low in flower, keeping temps in the ideal range in the day and at night, etc. Those are the hardest things to do as you might be fighting against building itself, weather and geography etc, which are not battles that can be easily won. In future, if you want to keep your plants smaller and spread out the canopy more to avoid light burn and increase the PPFD or what would otherwise be lower branches, I would recommend that you start practicing LST or SCROG'ing, but that's more a technique than an accessory - other than the plant ties you'll need to make LST'ing easy. 😁 Good luck!