Swim bladder/float issues in 12 yo Common

I post every 2 years or so with a new/unidentifiable issue with my one
fish, but I read more often and I appreciate the advise from the
regulars.

Here's my situation: I have one 8-inch 12 yo common goldfish in 29
gallons. It's unheated, but we're in Southern California, so he
hovers around 70-74 degrees (a little warm for comfort, in my
opinion). Marineland Emperor 280 filter/bio-wheel and a very large
circular airstone; light gravel and artificial plants. Ammonia is 0,
trace amounts of nitrates and nitrites recently (maybe I just need a
new test kit). Algae builds up enough to require a good scraping on
the sides of the tank every 4 weeks or so (too warm!).

Over the past several months, he started resting on the bottom. When
feeding time came, we would instantly energize and shoot to the top to
eat, but it seemed obvious that he felt "heavy" and it was a chore.
I tried to address any water quality issues (even though testing was
not conclusive) by adding some Biozyme and upping the changes for a
while, but nothing happened. I also increased the frozen shrimp in
his diet (usually eats flakes) thinking it might help a swim bladder
problem, and I salt after every change.

A few days ago, he started swimming around like nothing had ever
happened (aside from a red mark on his belly from resting for so
long). But, now I see the beginning stages of "floating" problems,
e.g. it looks like his belly is going up first sometimes, and it seems
like a struggle to get to the bottom.

So, I tried to feed him some peas, which he usually only eats 1 or 2
of at a time (and stupidly gave him a small piece of bran). After
looking around online a bit, I think I'm going to try the enteritis
treatment and fast him for 4-5 days with consistent salt treatments.
Since he's the only thing in the tank, I was just going to leave him
there.

I'd appreciate anyone's thoughts on what I'm dealing with here.

Some photos to see how the fish himself is doing:
http://www.flickr.com/photos/furriness/2494719639/
http://www.flickr.com/photos/furriness/2494719119/
http://www.flickr.com/photos/furriness/2494719807/

Thanks,
Furry