Water problems?

I'm having a water quality problem here. I thought I would list out
everything and see if anyone here has any ideas. I'm running out of things
to try.

I have a 40 gallon tank. Emperor 280 filter, I swap out the charcoal
cartridge in it every other week, rinse every week. The extra media tray in
the filter is filled with floss, which I also rinse or replace every week.
I have an airpump, and a 24" bubblebar which puts out a lot of bubbles
(bubbles do not interfere with filter pickup tube). I change 6-9 gallons of
the water every week, water is aged at least 24 hours in covered vat with
airstone, about 1.5 tablespoons of aquarium salt and declor added to water.
Besides these items, the only other things in the tank are some polished
river rocks, 4 small silk aquarium plants, 1 large silk plant, and 1 live
plant. I also keep 1 small piece of coral in there to keep the PH up. Hood
tank light, it kicks on at 3:30 every afternoon and turns off at 11:30 at
night. Tank temp is 76-78 degrees depending on time of day.

Parameters are as follows: Nitrite 0(zero) ppm;PH 7.6; Ammonia 0(Zero)ppm;
Nitrate is between 5 and 10 ppm (closer to 5 on chart). I use the Doc
Wellfish water testing kit for all of these readings.

5 goldfish in tank, 2 are very small (only an inch long, hatched last year),
one is medium with a 2 inch body, and two are large, about 8 inches overall.

Here's the thing: had high Nitrates (almost 20ppm) a few weeks back- I think
I goofed and overfed or something. It had been over 6 months since last
total cleaning, so I did one. Drained tank, rinsed and scrubbed the tank
with hot water. Broke apart and scrubbed out filter with hot water. Plants
and rocks were soaked overnight in bleach and water solution, rinsed over
several days and in several different vats of fresh water, and air dried.
Scrubbed the bubble bar with hot water. (scrubbing of everything is done
with old toothbrush that is only used for aquarium cleaning, and one of the
white nylon acrylic tank scouring pads).

Set tank back up, let it run with the filter going, bubble bar running, and
a hang on canister filter with fresh carbon for about 36 hours.
Re-introduced fish. Have been checking water every other day over past week
and a half and readings stay as listed above.

Okay, here's the problem. I have a large fish who keeps hiding. He gets in
the back and wedges himself under the bubble bar and sits on the bottom.
When the tank was first re-setup, he was active. As the days went on he
first started hiding when the light was on. Now he hides all of the time.
When it's time to get fed, he comes out and is active, eats, forages for an
hour or so, and then hides again. I also had one of my favorite fish die
last week; I don't think it was related to this (she had had a pea stuck in
her mouth 3 weeks ago and it was overnight before I found out about it and
removed it, and I think it lead to her death), but I can't be sure. Besides
the one fish hiding, none of the other fish show signs of injuries, fin
veining, or any other problems.

I think there is something wrong with the water that the tests are not
showing. Should I have replaced the bubble bar and bleach cleaned the tank
and filter as well? Are there any other water tests I'm not doing that I
should? It seems to me that there may be waterborne parasites at work here.
In the past I have had situations similar to this, and adding CopperSafe
medication to the water seemed to help for a while. I've done tests on the
water right out of the tap, and they coincide with the results I listed
earlier.

Thanks for any suggestions.

Devin