I vacuum my sand every water change as not to allow build up. Its good practice to disturb the sand on a regular basis.
Not sure what you did wrong, but ADA aquasoil leaches ammonia into the tank for up to three weeks, does nothing to fish if disturbed, and lowers your ph. All of these effects are proven and I have used this on several of my planted tanks and can vouch for each effect. Something doesn't add up.
-Zach
It's because of the fact that they're juvies. Feeding that often + Aquasoil + Disturbing the tank... recipe for death. That's why people grow out in bare bottom tanks, research would've helped here--I'm just commenting because I've used AS before and with the right knowledge it's perfectly fine. And with AS, if you do accidentally disturb it and cause a dust cloud, you'll have to do multiple immediate water changes over 24hrs or so.
So sorry that happened to you I use pool filter sand in my tank. Never had any problem I do vacum it often .