Re folks who take issue with hydroponics not being “woke”, not-organic, missing nutrients, etc…  It all depends on how you do it.  I use the Texas Tomato Food as a good base then, just in case, I add…

-Rock dust for minerals.  Very fine power can be blended into the nutrient solution.  Courser (1/16″-1/8″ pebbles) versions can sit in the bottom of your flood trays and tends to stay put.  You could also put them in nylon aquarium filter bags as a DIY mineral “tea-bag”

-Worm castings, also in nylon tea-bags hung in my main nutrient tank.  PS, this has the added benefit of stabilizing nutrient PH levels.  I have to use much less PH-Down (phosphoric acid) when using these tea-bags.

-Myco fungi and beneficial bacteria.  Great White has both in a water soluble powder.  https://www.amazon.com/Great-White-PRPSGW04-100049823-Mycorrhizae/dp/B00AU8JKR6/ref=sr_1_4?crid=3CTDA9WTZS7D7&keywords=great+white+mycorrhizae&qid=1579560771&s=lawn-garden&sprefix=great+whi%2Clawngarden%2C195&sr=1-4

I prefer my seed starting tray to be sterile with a little food grade H2O2. I reserve the myco+bacteria for grow trays.

FYI your hard to sprout seeds may do better with a humidity dome.  In your system, you can try a small glass (shot glass?) or plastic cap over the top of your individual net pots.  Covering your seeds with a bit of sand or peat moss can also hold in some moisture.

Haven’t had trouble with spinach in my biodome. I tried the pink celery you recommended and batting 1000, 2 of 2 so not a big test, but happy so far. They took 6-7 days and are just a 1/2″ or so. Pretty slow compared to tomatoes, tomatillos, collards, purple bok choy, and black goji. Those are all in the 2-3″ range already.

My garage gets down to the 40s at night, so it’s not the best environment, even with a heating pad the internal air temp drops into the 60s. In spite of this, the biodome has been bulletproof for seed starting but it’s too small to grow things out much.

They’re made by Park Seed https://parkseed.com/park-bio-dome-seed-starter/c/bio-dome/

I primarily use the 60 cell units because the rooting sponges are as inexpensive as it gets. Bought another 240 sponges last week during a sale, worked out to about 13cents each. In fact the sale included 60 and 120 cell Biodomes, going for $20 and $40 respectively. Almost tempted me to buy another one 🙂

The little 60 cell sponges, about 2″ tall, will do fine transplanted into your net pots. They’re pretty sturdy and will stand upright-ish in your net pots. Plant roots will have no trouble filling in the extra space.

My 40 cell unit is reserved for starting larger seeds like goya and watermelon because the sponges are larger but also about 2X the price. For reference, the 40 cell sponges are closer in size to your rapid rooter plugs but a bit taller at about 2″.

The sponges are are actually sturdy enough that I’ve *sometimes* been able to reuse them after a trip through my worm bin to clear out the roots.

The Biodomes are also sturdy, I’ve had my 3 units for 7+years. My only complaint is about the hard plastic dome lids. They can crack/chip if you drop them hard enough.