I planted a few of them under a 150 watt hps lamp and they've been doing great. By now, each plant has a few "true" leaves.
However, I've noticed that on two of the seedlings, the cotyledons are showing some discoloration around the edge.
Here is the worse of the two: (the color is messed up because of the hps' limited spectrum)

Ignore the scarring in the center of the cotyledon, that is from a small insect problem that I've taken care of.

I'm worried about the possibility of it being a fungal infection that I wouldn't want to spread to the others ( I only have so many seeds). However, as it's only developed on the cotyledons, could it just be the plants' natural progression?

Can anybody identify this? Am I being excessively worried?

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