goldfish question

Hi everyone, new to the group.

I have a feeder goldfish, about 1.5" long, very healthy appetite and
energetic when it wants to be. Hey gotta start somewhere right? I
picked up a 3-gallon Marineland Eclipse 3 with the bio-wheel filter
'cause it got pretty decent reviews on some sites I looked at and
decided anything with a filter in it is better than (gasp) a bowl (heh,
i read the big eruption about the lady who understandably didn't want
to pay $500 for pet deposit to keep two small fish in a bowl), bought a
5lb bag of gravel, rinsed it all over, set up the tank, let the water
sit for a good while, got some plant bulbs but after 2 weeks nothing
has grown so stuck a dozen stems of anacharis in there too, got a nice
decorative rock with a hole in it, and my goldfish loved the tank and
swims through the hole in the rock, etc. I feed sinking pellets in the
morning, and a few flakes at night. Water was tap water treated with
AMQuel+ (nitrite, nitrate, ammonia, chloride, chloramine), and is
typically 75F without a heater installed.

I've cleaned the water in the tank after a week (changed about 25% of
the water) using a typical gravel cleaning hose, worked great.

All of this sits on my desk at work. Coworkers love it, they ask lots
of questions prompting me to go learn more. My wife has a 10gal
freshwater tank at home and she's been a good source for info (she's
kept a tank for many years)

Few questions/thoughts I have:

I put a 3-day disolvable feeder in the tank Friday before I left work,
came back in Monday and saw it was mostly dissolved, which meant the
fish was well fed - but the water was pretty murky after that, which
prompted the 25% water change which I was told I only really needed
once a month, not what will turn into a weekly chore if I keep using
the 3-day feeders. A pet shop staff member said that goldfish are
pretty hardy and as long as they're normally fed on Friday (not
overfed), they'd be fine until Monday morning - I've seen other sites
suggesting that making your fish fast now and then is healthy for them
- is this true?

For the last two days though, and again today, he's been content to
swim around a little, then rest right on the gravel on the bed of the
tank. he/she/it flattens its dorsel fin against its body, and just sits
there. I know it's breathing 'cause every time it happens, I watch
intently to see what's going on. After a few minutes, it'll swim around
a little, might hide up in the top-back-left corner of the tank behind
the filter tube, but still drifts down to the bottom a few times a day
and just 'sits' on the gravel. I can't imagine that's normal behavior
for a goldfish... He doesn't tip over, he's not swimming crooked or
anything, he's perfectly vertical while swimming around, but he just
sits right at the bottom of the tank. Very odd in my opinion, but then
I'm not anything close to an expert so figured I'd ask you guys.

This morning, I noticed a few little white spots on its right side near
the anal fin, but as the day went on they seem to have gone away. He's
been swiming fine, eating fine, loves to siphon the gravel for
leftovers, but then he'll pause, stop, sink to the gravel, and sit
there. There are no other spots on him, his scales are bight shiney
orange, his fins look okay (no red, no rips etc)

Is this really normal behavior, or should I just consider that a $0.20
feeder goldfish really isn't meant as a long-term aquatic pet? Should I
look at getting a pair of tetras or something else instead?

Thanks,
Ian