Lethargic older bala shark

Question
Dear Lauren,

I have a 40 gallon freshwater fish tank and have had it for about 3.5 years. I had 2 bala sharks that were buddies but one of them unfortunately jumped to his death while we were moving.
So now I have 1 left, and he's been getting increasingly lethargic. He's been without a fish buddy for about 3+ months and my girlfriend recently bought me a surprise gift of 2 baby bala sharks to help keep him company.

The bala in question has been hanging out in this one corner, by the plants, just hovering there and sometimes comes out for a swim. I'm not sure how old he is, as I inherited the tank 3.5 years ago. He's about 6+ inches.
I've noticed his scales aren't as shiny as they used to be and there's some white residue between them it seems. I only just recently noticed that.

This morning I noticed he was kinda lying on the aquarium floor, not looking so hot, not jumping out of his skin when he sees me coming. Normally he's a nervous guy, but today he just sorta sat there and slightly twitched as I tapped on the window to get his attention.
Honestly, I slightly expect him to die today or tomorrow because he just doesn't look right at all, but I was wondering if there is anything I can still do?

He still eats, the temperature is in the mid 70's, I just replaced the filter and did a 30% water change a week ago.

What do you think?

Answer
hmmm..sounds nothing like any illnesses...more like old age...
here is a website..of all diseases, what causes them and how to treat them (and prevent it)

look for your fishes symptoms...im sure you'll get the idea when you click on the link =]

i hope your fish will be ok
=]

http://www.fishdeals.com/fish_diseases/stressdisease.shtml