First I would like to point out that any diagnosis with the info provided is almost impossible. While it does point to a suppliment problem, it could be number of other things as well. I agree supplimenting everyday is a good start but I would watch closely incase there isn't something else going on as well.
For starters, do you have a pair or two males/females. Have you noticed any fighting between the two or is one being bullied ? At 10 months they are starting to mature and you may be seeing the start of incompatibility. This would stress one of the frogs and when opening the lid to feed or mist, the additional stress could be causing seizures.
Does the frog spend an excessive amount of time soaking in water. (sure sign something is wrong)
What are the temps/humidity/ventilation in your setup ?
What is the frogs overall appearence ? Is it eating normally, any weight loss, is his skin moist and shiny or dull ?
For something like this, I would seek the advise of a good vet that has experience with dart frogs. I have heard good things about Dr. Frye posted on this forum, he deals directly with you on the phone and ships out meds as needed.You may want to give him a try if his seizures continue.