I'm sorry for your loss -- Even though everyone always says not to, I've taken on several sick Leos -- I've definitely almost been in tears watching my current "donation" struggle through courses of anti-biotics, baby food feedings, daily electrolyte "baths" etc. I have to commend you for seeking so much information and educating yourself, so hang in there!! Sometimes there's nothing you can do for the sick lil' ones.
This may have been asked already, but where are you getting your babies from? Perhaps your seller/breeder/petstore is selling you Leos that already have parasitic infestations, etc? Also, its better to pick a flighty lil' bastard than a "sweet" baby that tolerates handling. All my babies have been psycho but all calm down eventually and are happy to chill on an arm or shoulder while watching TV. It just takes time.
If you do opt for a new pet in the future and you can interact w/the babies before picking one, ask to see them at feeding time and pick a nice fat one that's more interested in its food than any of the others.
I hope this isnt rhetorical and you've heard it 200 times but perhaps try another source for a new baby Leo after the "mourning" period and make sure you cleanse the tank thoroughly, new substrate, everything. If you're tying to use bleach, I'd be VERY careful -- try one of the commercial anti-bac gels/cleansers and make sure to get ANY residue left over from the cleaning products out (I do this w/my feeding tanks in the bathtub!)
Good luck & hang in there,
~Linz