Can't believe no one has suggested this yet, but TAKE THEM TO THE VET!! A vet visit might have saved the one that just passed. If your other ones are bad off these remedies being suggested work great, but we aren't doctors and can't tell if there's something going on internally. Also, prescription meds may end up working better or being more appropriate. Please take them asap!
And THANK YOU for trying your best and rescuing them. Hopefully you won't lose any more and the remaining ones can make full recoveries. Good luck!
I didn't know until after my pet store visit just now that there were any vets that would check them out in my area.