Hi and welcome to this very fun stuff. I'm sure there are more expert folk out there but since I've been dealing with root issues for months now, probably I can make some suggestion that might help.

As for the light, if it is not pinned up for the photo, it might need to be closer to better provide for the plants energy requirements. But in the seedling stage it won't be as big a deal. With regard to the root size, it seems to take me around two months to successfully transition from scraggly root systems to more robust ones, and that is with what I believe to be an optimal and custom fertilization profile and for plants that don't have a very weedy growth profile, but I didn't notice dandelions or arugula in yours.

The real issues you are probably already wondering about include whether it was a good idea to start them directly in the system or have some sort of nursery or plug starts. That's easy, the starting plugs work great and hydroton in my experience is a little tough on small plants. That said, what's done is done and if you make it over the hump there's no reason the plant can't take off growing better.

First thing you mention is you've got the net pots submersed about 2 inches below the reservoir surface. While there may or may not be a one size fits all statement, it just seems way too deep to me. I've done well just with my bottom hydroton scraping the surface as well as receiving lots of splashes from the aeration bubbles popping and jumping up the roots, which they do with nutrient solution and all, not just evaporation.

Possibly, the plants are too wet. This will promote less oxygen rather than more, more fungal and microbial growth, and generally more runty plants, if they don't rot first. Not saying any system could work on the appropriate maestro ;), only that being drowned in water could be responsible for your worries. On the other hand you just may have greater expectations and might need more patience. Unless we actually could see a root/root system if you are lucky, it is hard to say anything from the pics other than it might have been easier to do a nursery first for the seedlings. The oxygenation works by capillary action, meaning there is no need to keep the water level so deep or it can get waterlogged and loose oxygen, which would be a bad plant environment

Another design is just to do what you are doing but have the pot above water somewhat, and run a top fed drip irrigation hose in the same sort of system, just one mini needle poke-able hose that passes thorough everything like a winding thread, and still with bubbles erupting at the surface. That particular design is known to yield some of the fastest obtainable growth initially, which can be very satisfying to our motivation even though three months later everything might end up the same whether you did that or not, although you will get a couple of weeks jump on production.

Best of luck!

As for setting the pH only once, that is more of a continuous process that I personally do every day for the first 3 days, and then I let it drag out much less frequently until toward the end when changes become abrupt and nutrient is so far eaten that it is out of balance and the pH can make big changes from one day to the next; when I'm tired of that and think the plants are living on residue and plant poop more than high gain fertilizers, I change the whole thing.

I hope you get something out of that, but also recognize so much of plant culturing is personal interpretation and the plants themselves are often much more versatile than we give them credit for, able to grow and even flourish in different media at all stages even though standard might be in grow plugs.

I agree with all of the previous comments, including the lighting must be much lower to the plants.
I have attached a photo of some seedlings about 2/3 weeks which only in the last couple of days have thrown out some long new roots chasing the nutrient solution.
From this photo you can see that only about 1/4" on the bottom of the pot is submerged allowing the roots the follow the solution as it is used up and most importantly allowing oxygen available to the roots as this solution is lowered, or the roots would drown, rot and die. I hope you continue and enjoy hydroponics.

thank you both for your thoughtfull posts. they have really helped. my system is now adjusted and plants are growing fine. it guess it's a little bit of trial and error at the beginning, but once you get the hang of it, it works nice.

adjustments:

1. light is way closer to plants
2. added 1 tspn/gallon of RapidStart to enhance initial growth
3. water is being changed every 7 days