Seedlings look dried out but had enough water ( first time under LED)

the waviness could be caused by pH swings, but also early leaves often are not representative of what the leaves will look like later on.. this issue may go away on its own if you have a well-balanced feed, stable pH and loosely controlled environment.... fairly low bar. se700 is a serious light. 2045 umol/s according to spec sheet. Over 18 hours you should have this high enough to cover roughly (2045/600 = 3.4) 3.4 m^2 or 180cm x 180cm, if not you are potentially giving too much light... or, closer and dimmed appropriately.. .more than 1 way to skin this cat. *technically thse shape of best light footprint will be a rectangle due to how light radiates from a light with parallel bars.* but what i saw here expresses the gist of light intensity. You only want about 600PPFD over 18 hours.... when you switch to 12/12 you need 150% of that, or 900ppfd. If you supplement co2 you can go another 33% higher than that for each context... this is meant as a ballpark idea and you'd need further trial andn error to dial it in based on plant behaviour and reacting to hit ... specifically distance between growth nodes will be best way to determine optimal intensity... as well as observed growth rate over time of course.... they will coincide with each other. so, remember you need 66% over 18hours what you need over 12 hours... easy to damage plants with powerful lights if you haven't used them in early vege before. FWIW, these look fine... as long as those internodes don't remain incredibly tight, it's probably fine. this will help yo after 12/12 too to adjust lights properly... whatever tehy handled over 18 hours, you can give 150% more over 12 to match it. it amounts to the same DLI provided (daily light integral) ... DLI is best way to udnerstand this because size of garden and hours of use are irrelevant to this measurement and apples to apples comparisons can be made.