Good luck with your system.  I'm about to break ground on mine after a couple of years of reading, scheming and saving.  Lots of work to do but I hope to have fish in a tank by Nov.

regards

GT

Re:  charcoal dust on the roots .  Sorry for the slow response Joe - I just didn't see this.  It's possible, although the dust can easily be rinsed away before putting it in the system.  I can't speak to it with any certainty at present.  I've just started purchasing the components of my system and will not likely incorporate DWC anytime soon.  Will start with media beds and add short towers next.   I now suspect that charcoal in an aquaponics system may act too much like a nutrient sink.  However, that doesn't seem to occur in a dirt garden so maybe not.  Eventually I may incorporate it in some fashion, possibly a media bed full of charcoal.  Happy gardening.

Re: charcoal powder sticking to roots

The powder is easily rinsed away after crushing. Possibly a screen could be used for rinsing which would also grade the charcoal, leaving only particles which are at least large enough to suit your purpose. Even if small particles attached to plant roots, I doubt that would hurt the plants. In soil, I've seen plant roots attach themselves to charcoal. Essentially, it is inert, porous carbon, except for that which is absorbed, plus bacteria taking up residence. I became convinced that it is beneficial as a soil amendment but have no experience using it in aquaponics. It wouldn't cost much if anything to try it and you could probably determine beforehand whether it is likely to stick to roots without actually introducing it into your system.

At the moment I am looking for an alternative to vermiculite substitutes for seedlings.

You might want to try charcoal, if you haven't already. I've been experimenting with making it and using it in soil and, although I haven't tried it yet, I believe it might make a great seed starter medium, possibly in the gravel bed itself, or floating raft of some sort. For internet lingo, try terra preta or biochar, bio char. The simplest method I've found for making it (and I've done it many ways) is to build a wood fire, burn it down to coals, and extinguish it with water. Crush to any consistency you like. An easy crushing method is to put a few inches of charcoal in a 5-gal bucket and crush with a flat shovel. Metal against plastic works well for crushing but wood on plastic is more difficult. If you have any questions, feel free.
Regards
GT