Might be a classic case of "damping off" with one strong survivor...To wet's no good for seedlings...Sorry to hear that...

When I feed a seedling varies from strain to strain,but my babies are already sittin' in a decent amount of worm castings so I know they're not starvin'...I usually feed em very lightly,(1/8 tsp GH Gro and 1/4 tsp SuperThrive per gallon) when second internode is well established...