Why are the tips of my leaves turning brown and crispy?

That's classic nutrient burn — the tips burning first. If you're feeding nutrients, cut the dose in half. If you're in hot soil (like FFOF), the soil itself might be a bit strong for your seedling.

Just water with plain pH'd water for the next week. The burnt tips won't recover but new growth will be fine.

Tip burn is the most common issue I see from beginners. It's almost always from feeding too much too early. Cannabis is way more sensitive to overfeeding than underfeeding. When in doubt, feed less.

Check your runoff EC/PPM too. If it's significantly higher than your input, there's salt buildup in the soil. A gentle flush with pH'd water can reset things.