This brown spot is being caused by sitting in too much water for too long. Just one too many times can cause root rot which causes death up the line. The calcium on the side of the pot.
Your plant should stay in the same soil for the next few weeks. Give this plant a balanced fertilizer and use half of what the directions say. I would use distilled bottled water.
Water thoroughly and then pick it up. Feel the heft. Do not water that plant until it is obvious the pot and plant and soil are far far lighter. Then water deeply again.
Let's find out what the problem is first and then we'll be able to help with remedies.
Your plant needs a little bit of balanced fertilizer (not blood meal or bat guano or worm castings or raw compost of any kind). Osmocote 14-14-14 All purpose extended release is my favorite for newbies in gardening. Use HALF the amount in the directions and depending on the health of this plant, the amount of sunlight it gets, plenty of air in the soil...you might get by with just one application per year. Especially if this plant is to remain indoors?
Looking at your plant shows it needs basic NPK, this plant has had too much water and not enough light. You need a brighter spot unless this is a curtained south facing window? Repot this plant after 2 weeks in sterilized potting soil medium. Wash and scrub the pot, knock off excess soil and cut off dead/browned/mushy roots if there are any. Transplant into fresh potting soil, water and do not water again until that plant/soil/pot feel very light.
Raise the bottom of the pot off the surface it is sitting on...such as the saucer? by 1/4" using pieces of ceramic tile or something that keeps the pot stable. Never allow any plant to sit in water for more than 10 minutes...or whatever. Do not water again until that pot/soil/plant feel obviously lighter.
There is something I am missing. Was that soil purchased in a bag with a label? Do you remember any additives such as gels or sponges for holding onto water? Did it come with fertilizer? Was it sterilized? If so you wouldn't be having troubles with fungus gnats or fruit flies. Curious. I am thinking that the soil might have pH beyond or below the proper pH. Have you used the water in your fish tank to water this plant? I am assuming you might have a fish tank with the gravel?
I just realized the plant as the model for leaf signs, in the link I am sending, is Cannabis. One of the greatest things about legalizing this plant is how it is making great gardeners who are honing their gardening skills, skills that will work no matter what crop they grow. I hope this is okay? They'll let us know if not, we've great moderators!
Check the pH of that soil. The chemistry used in aquariums needs to stay in the aquarium because it will definitely mess with the chemistry of the soil negatively.