Did you look at any roots? Color? Anyhow hydroton drains really quick, its also what i use, so i flood 6 times a day for 15 mins. Rockwool in a friggin sponge so 3-4 times if its a smaller cube but usually 3 times a day for 15 mins. Peroxide should be used at about a tsp. a gal, but i have heard of 2 1/2 tsp per gallon. Oh thats for 3% h202 if you use food grade 35% you would only use 1/10th of that. Overwater does cause root rot, and this was the first way i learned about it. Unfortunatly i never saved my plants, so you need to catch it quick. Hope all goes well for ya though.
. I really hope not though, see if they are bright white and healthy or if they are brown and a little slimy. If so you need to add H202 fast. And maybe an enzyme. Again i really hope this isnt what it is. Also nutrient burn can cause it, maybe try adding a little less if they yellow add more. Adding silicon helps the leaves retain water. I assume your using hydro. What kind of system are you using?