SEEDLING PROBLEM, HELP PLEASE!!!!!!

hey all, ok im a noob with growing so this was my first grow. basically my plant germinated fine and i planted it 18 days ago and took the tent off when it sprouted above ground. everything looked fine but about 4 or 5 days ago the top leaf turned yellow and now the bottom one has turned yellow with the tip being brown and it doesnt seem to have grown at all since the yellowing.

i planted it in a a large pot so i could avoid having to transplant it and used garden peat soil by the manufacturer "gem". since it sprouted i have had it under a 240 watt HID light at about one metre away from it for roughly 16 hours a day. a few days ago i added the tiniest amount of fertiliser (4, 4.5, 8) but it was such a little amount so i dont think that is the cause, as the yellowing had started before then. i've been pretty careful to not over water and the soil has been kept just moist throughout. iv had the temp vary between 65 and 82 farenheit but usually around 77. humidity has been between 30 and 60%.

i don't know if it's this pythium wilt thing i just read about but i have a feeling its heat stress or burn because at night i've had it in my room with an incandescent bulb on it at around 25 cm away for a few hours before i went to sleep which may have caused the problem so i stopped using that once it was inside the house about 3 days ago. i know the cotyledons are supposed to turn yellow and die but this seems like it's happened way too early as the plant is very small.

i've included a few pics, sorry for the quality but you can make out the top and bottom leaves being yellow and the tip brown with the middle leaf still green. any help would be much appreciated. let me know if i can be more specific, thanks. :(

it doesnt look like you have anything for drainage in your growing medium whasoever and you will definitly need it especially in peat. i dont know if this is your problem or not but even if it isn't you should still add a whole bunch of perlite. im not the most knowledgable person on this site by far but this is one thing i am sure of.

ok i will do that. thanks, it may be the problem i dont know but i will definately get some anyway.

You shoudl add some vermiculite too. If the mix is peat-based used 1/3 of each: soil, verm. and perlite. You can always add more perlite to that mix as well. Good luck.

i dont see a stem leaf or anything in those pics; did you not plant the seed in soil after it germed? Maybe someone sold you carrot seeds and beat you of your cash? Im really not sure what that thing is, & im seriously not trying 2 be a dick about it but i really want to know now.

Id think fert/nute burn & low quality soil may be to blame for this hideous abomination.

i planted it straight after it germed. lol no i bought it from a very reputable local shop so i'm sure its not a bad seed. it's definately not fert burn and that particular soil is used for all kinds of plants.