Joy,
Sorry for the troubles.
When I had to make that decision I went to my regular vet & they did it for me.
They were very professional & understanding & I stayed right there until the end.
Probably your best bet.
As far as the tank, I've been using a Stainless straw to suck it out during a water change & it's better.
The only tang I know eats it is a Naso.