The most likely cause of Gizmo's sudden weight loss and lack of energy is kidney failure. We've been advised to add a vitamin supplement to the water, and feed him more fresh vegetables as this will make things much better for him.
He seems as healthy as he can be under the circumstances... his heart is strong, his chest is clear, his scent gland isn't blocked and his skin is healthy. But should he get in pain or his condition become uncomfortable, we have been advised that it would be kindest to him to let him go.
I say it was better than I'd expected, because I thought I was going to be told that we'd have to let him go anyway. I was in a bit of a state about him last night.
Should we lose Giz, I'm not sure we'd get another to replace him... for one, I doubt that Ash would appreciate a new tank-mate.
So really, for now, it's just important that we give Giz and Ash the best we can for as long as we can.