you have no need to flush... flushing should always be a last resort, unless you overfeed... which doesn't seem to be the case.
if they continue to detoriorate, more likely due to stripping soil of nutrients from flushing.
those first few sets of leaves often are not as pretty as what the plant will be a few growth nodes later. single blade and cotyledons often get sucked dry by plant, although this early would be a bit odd.
it's a soilless grow.. should be 650-750ppm everytime you irrigate at this point. get 10-15% runoff. let top layer change color before repeating process. by getting the runoff, you maintain pH and concentration levels in substrate throughout wet-dry cycle. If you want some ideas for PPM of individual elements, check out any of my diaries and there is a table of what i use. NPK should be similar, but Ca, Mg and S depend upon your tap water. i have hard water, so you may need more of those than what i use. go watch some vids by dr. bruce bugbee on youtube about substrate and feeding... will help.