The drooping might be caused by the light being to close, if possible raise it up. you should see the leaves start to reach up.
Do not add liquid CO2 to your water!!! DO NOT !!!! CO2 is for the leaves to absorb, and only during lights on. roots need oxygen.
Since you are using
smart pots (cloth) your water will dry out more, this is ok...better than not drying out. you can check your pots by lifting them,
when mostly dry they will be lite... after watering they will be heavy.
And you have seedlings, so you don't want to water, as to get a run off. your roots have not spread out far enough to absorb all the water. that is why your tester is reading that way. it will be moist towards bottom..no roots there.
Your ph should be 6.2-6.5
In veg it is best to have a higher humidity than 30-40% if possible raise it some. around 60-70.
I wouldn't turn the soil any, after your seedling has grown some, you will get the really small capillary roots that spread out just below the surface.
if its drying out to quickly, its because of low humidity, strong light, and cloth pot. you can put on a top layer of mulch, that will help maintain surface moisture.
And the 400 watt light is more than enough for your tent size.