Okay cool thanks everyone, I've noticed two are starting again to grow so I think flushing out the nutes and maintaining the proper ph is helping. Is it normal for ph to raise from 6 to 7 over a few days? I wonder if hydroton leaches stuff into the water?
and my third and more severely affected seedling (pics 2 and 4) still looks horrible, I wonder if it will even recover. Is it wise starting more seeds now to grow them staggered multiple weeks apart? Or will that mess with my potential flowering cycle and whatnot?
excuse typos, on my phone
I'm glad jackmc was able to step in and answer your question about the PPM's. Listen to what he has to say, give it a go and report back.
Regarding the hydroton (clay pebbles or whatever you want to call them) YES it does raise pH!
This is because they have a naturally high pH. If you put half a bucket of water, test the pH, add a few handfuls of hydroton/clay pebbles and test the pH again, you'll see... most likely... the pH has risen noticeably!
What you need to do before using hydroton is WASH it and get all the dust off it. Wash it and then wash it some more. I have a mesh waster paper bucket like you'd see in an office that I put my stones into and wash them that way. You could use a plastic laundry basket or even a kitchen strainer for smaller amounts.
After you wash your hydroton you want to SOAK it... generally I will soak my hydroton in pH 5.5 water for 3 days before I use it. Although I read recently you should use even lower pH water for soaking it.
EDIT: You want to check the pH of the water in the bucket your stones are in and if it's raising above 6.0 you want to add a touch more pH down. You want it so if you add water, it sits level at around 5.8. Anywhere between 5.5 - 6.2 is fine, but 5.8 is in the middle and it's kinda the 'safest' pH to aim for in hydro. /end edit.
Now you can't really do feck all right now because your plants are already growing, you're just going to have to keep adding pH down. However don't pH your water more than once in any 48 hour period. I would pH it down to 5.5 and let it drift back up to 6.2 and repeat that every at least 2 days.
Hope I'm being clear in all of that, any questions and fire back. Cheers! p.s. glad to hear you're making some progress. It's okay, lots to learn the first few times but once you have it right I swear to god they'll explode in size. If you keep posting lots of info here I'll keep helping you. (private message me if you don't hear back from me quickly enough)
