Looks like nute burn to me. I skimmed through and didn't see what type of soil they are in??? If they are in anything other than seed starting mix, there are certainly some nutes already in the soil. I start in jiffy pucks and transplant into soil and almost never have to feed before they're transplanted into their final homes. Smokemaster is also on point with his recommendation for a fan on or around the grow area. I point small fans directly at my seedlings which does a few things...it prevents still air/humidity problems such as fungus and edema but it also strengthens their stems which prepares them for the winds they will face once transplanted.
As for the bottom watering...it is the only way to go. Depending on what soil you use (especially organic) top watering will wash down or float up everything that is important in your soil...so, the nutes will go to the bottom, while the perlite or vermiculite floats to the top. Organic soils with castings and manures as their main nute content end up a soggy mess at the bottom of your containers while the top will seem rather dry after just a couple weeks of top watering.
I tried the solo cup routine one year...and they can work, but they're very tippy and the drainage is tough to get right. I now use small nursery containers that fit perfectly into propagation trays. I then graduate them into bigger containers designed to fit 8 into the same propagation trays...with this system bottom watering is a breeze! I know things can get spendy, when growing, but these are pretty cheap, can be used year after year and are much easier to use and work with...here is a picture of what I use along with prices...
The small containers are 25 cents and the tray was a dollar...this whole setup is $5.50 per tray.
Sorry was a bit long winded.
Above all...stay super
chilled on here and folks will certainly rush to your aid. Can't let little stuff like minor thread detours bother you or you'll find folks much less eager to help out.
Good luck! Keep us posted!
Shane