*HUGS* They were so beautiful, oh I KNOW how heartbroken you are! I wish I had something to add here, but BDM and Chuck pretty much covered all I can think of, too! I'm no pro, either. . .
Leave any leaf that you think looks like it stands a chance of rebounding, but prune off all the dead leaves *cries*- you want to get them out of the tank, they aren't doing any good in there
I also use [ame="http://www.amazon.com/Seachem-513-Flourish-500ml/dp/B00025696M/ref=sr_1_1?ie=UTF8&qid=1375530503&sr=8-1&keywords=flourish+comprehensive"]Flourish Comprehensive Complete[/ame] liquid ferts by Seachem, so second that recommendation - its good stuff, and has everything the plants need in a good balance. The same company also makes [ame="http://www.amazon.com/Seachem-Flourish-Tabs-10-Count/dp/B000255QLG/ref=sr_1_1?ie=UTF8&qid=1375530554&sr=8-1&keywords=flourish+root+tabs"]root tabs[/ame]. I linked to Amazon so you can see an image of what we're talking about, but both of these should be really easy to find anywhere - PetSmart carries both, as does almost every fish shop I've ever been in. . .
These are great products, but since your plants have been doing so well up until this point, I have to assume that you've managed to find a good balance in your tank, and that the ferts you have been using, along with the substrate, and fish waste has been sufficient to get the plants what they need. Which is FANTASTIC!
Except that it doesn't help us figure out what went wrong.
I've seen similar looking leaves on one of my swords when the lights in that tank needed to be replaced. Even though they aren't burned out, tank lights start to lose power after about 9-12 months or so (I shoot for 9 months on my tanks, but everyone has their own way). If the lights haven't been changed since start-up in December of last year, they're probably due . . . but with my sword, it was a gradual process, and not nearly so extreme! So. . . more good advice, but if this happened all of the sudden after one water change, and everything was perfect until then - lighting doesn't explain such a sudden and dramatic decline in their health, either!
Temps are fine, the plants are established (I'm with you! HATE that acclimatization process!), buckets kept clean. . . I'm leaning toward Chuck's theory here - I know I've grabbed the wrong bottle by mistake before, and also have come *THIS CLOSE* to forgetting to de-chlorinate altogether!
We all make mistakes. If your son accidentally didn't dechlorinate at all, that would
effect the fish - but the plants would probably have been fine - chlorine doesn't bother them (crypts would maybe be a different story, but I doubt you'd see melting from swords) UNLESS he somehow grabbed a bottle of something else. I hate to suggest that such a thing happened, but it could fit - depending on what might have been added to the system.
Otherwise. . . *grasps at straws* was anything done around the house that day? Any unusual chemicals sprayed? Maybe carpets were being washed, the house bombed for insects, or any painting/construction projects going on? Even perfumes or hair sprays? A bit of a stretch, I'm just trying to think of anything. . .
I'm going to see if I can point some other members in this direction in the hopes that we can get some new thoughts on what could have caused this. I'm really sorry I can't be of much help. I just. . .can't think of what it could have been! Whatever it was, I hope most sincerely that it never happens again! Hopefully we can figure SOMETHING out!
P.S. I'm also a sucker for black sand, the 60 looks like it's on its way to being gorgeous. I know I'm not alone when I say we'd LOVE
to see pics of your tanks! Now that you've decided to come out of hiding you should share! We're not going to let a fellow fish geek go back into hiding without a fight!!! ^.^