This is going to sound harsher than I mean it to but here goes.
It's his fishtank. I understand that it's frustrating seeing his unhealthy fish but when it comes down to it, it's his tank and his fish. You've offered him help in various forms and he's declined all of them. Sneaking behind his back isn't going to help a thing. It's not going to teach him and, if caught, could very well peeve him off. Remove yourself from the situation. I'm hoping you don't live at home so when you do visit, don't look at the tank don't talk about the tank, just act as if it's not there.
I view it this way, he's not going to take/get help until HE wants it.