Fish problems. Dropsy?

Sorry if this comes as a second post, but I'm a little worried, and
I've been off investigating!

Right, stats first:

Temperature is 21'C-22'C (72F) - I'm buying a heater tomorrow.  Info
on how they work make/models recommended would be helpful
Nitrites - 0 (not sure if I believe that)
Nitrates - 40 (water change time?)
PH 7.8-8 (steady)

Tank is definately overstocked at the moment.  Due to my financial
situation, I'm opting for regular water changes.  Goldfish & shubunkin
have been growing expotentially.  Now 3" (1" in 1 month) Tank size is
11g and was only temporary.  I'm worried about upgrading for 3
reasons.

1.) Top floor flat (3 stories) - floor is wonky and I'm not sure it'll
take much more weight.

2.) Only just bought a new hood for the last one.

3.) will need replacement hood, tank, light bulb & stand.  I don't
have any more room!

I suppose the good water quality is the reason for the sudden growth
spurt.  I did sell my old hexatank (a whole 1g) but I suspect I can
retrieve it.  I still have my new temporary tank (I was going to use
it for brine shrimp, but it's completely impractical for that use.  It
is less than 1g (recommended by most LGF as a "perfect starter tank". 
I was keeping it for plants and for emergency dips etc.

History:
Fish related - fish swallowed stone about 2/3 weeks ago
Enviromental - change from gravel to pebbles about 2 weeks ago
Enviromental - several water changes, addition of salt (probably too
quickly, but nitrates seem to indicate that the biobugs recovered OK)
Fish related - Fish swimming around at bottom, bit slow.  Halved
feeding and alternated between brocolli (bad idea, v.smelly water) and
bloodworm.  1 week ago.  Fish on high protein at the moment (no peas
till today)
Enviromental - Finally realised the reason my pump kept stopping after
I turned off the electricity was because small yellow clog meter was
firmly encrusted.  Washed pump thoroughly in used tank water.  Lots of
crap came out
Enviromental - water change, probably a bit to cold (winter here in
UK, I try and let water stand for 12-24hrs to get to room temperature)

Water is conditioned with a declor when I don't have time or change is
needed urgently.

Conclusions:

I need help here.  I understand that this situation is probably my
fault, but the fish seemed to be doing really quite well in their
enviroment (all growing, no illness for six months).  Stress & water
temp is probably to blame.

All and every bit of help would be appreciated.  I really don't want
to loose my chubby oranda, especially after he's only just started
developing his headpiece :- )