It is 'possible' that it is natural decline but does not appear so when compared to natural cotyledon decline in the many plants I'm familiar with (never grown this one).

It is also 'possible' that it is leaf scorch from the lights as it has that appearance but usually shows on true leaves, not just cotyledons.

But the most probable IMO is pest damage. Aphids will do this but so will spider mites and fungus gnats.

As to a fungus disease - assuming you are not keeping the soil too wet or the humidity way too high, the odds are quite slim. Especially given it is inside and the true leaves would also show symptoms. Soil fungus develops inside but leaf fungus is rare.

Can't see the true leaves in the pics but if it were my plant and IF the true leaves do not have these symptoms I would removed the cotyledons and dispose of them carefully. They are no longer needed by the plant anyway. Then monitor for pests very carefully and treat accordingly.

Hope this helps.

Dave

Hey Dave, it's been a while. I just wanted to let you know, you were right about the pests. I think I had gnats, but I did away with them, and now my plant is taller than my leg and doing wonderfully. Thank you very much for taking the time to help me out, man.