I know its a little too late but sorry for your loss. i had a similar situation happen to me before. I tried everything that you did, just when i was about to give up on my 6" xingu rhom i remembered something a pet shop owner told me about nitrates.(if you do a water change and you overclean your tank theres a strong possibility that your nitrates me be to low). so as my fish laid there stuck on the power head i grabbed two dirty filter bags from my filter and shook every partical of waste off that i could. within in five to ten minutes my fish was back to normal. as crazy as this might sound it worked for me. i wish that i saw your cries for help three months ago.maybe your big guy would still be with us.