Sudden Death

I am fairly new in the aquarium world,, but have been reading tons of stuff to try and do things right,,,,, I have a 55 gallon freshwater tank that is now 5 months old. My water quality is good and consistent,, althought slighty more alkiline than I would hope for the fish that I am keeping. My parameters are steady at:
7.5 pH, 10-12 GH, 0 NH3, 0 Nitrite, tank temp is 78° F
I conduct weekly water changes, usually 12 to 15 gallons... and the tank is moderately planted.....
At one time,, I had more fish than I do now, but,, like I said,, I am trying to do this right,and I am putting all new fish through a three week quarentine.
for the past month,, I have had a Powder Dwarf Blue Gourami, and four sailfin white Tetras....
They are quite peaceful.. and I have no conflicts among them..
I feed them two or three times daily,, just a few flakes of dried flake food at a time,, and about every other day or so,,, I take some time and feed them frozen brine shrimp,, only as much as the consume immediately..... just last night,, my wife and I were looking at the tank commenting on how great the Gourami looked,,, it's color was brilliant, and it had great body tone and beautiful fins......last night it appeared to be making a bubble nest,, in fact it was,,, this is the thrid time it has done this.....
this morning,,, when I woke,,, before the lights clicked on, I noticed that the Gourami was swimming virtually vertical,, head high. I quickly ran a complete set of water tests,, and all parameters fell into the ones listed above.. I removed the Gourami, and placed him in my quarentine tank,, and within an hour he was dead..... I'm totally baffled......The Tetras look as great as ever, with a hardy appetite,, and beautiful flowing fins.
The ONLY mistake that I may have made was,, about four days ago, I purchased a few more plants, and left the lead wrapping around the bases of them when I placed them in the tank. This morning, after the fish died, I removed the lead wrappings from the plants,, and did a 15 gallon water change,,,,, I would appreciate some comments... I would hope that the comments would include any known sensitivities that Gourami's may have. All read about them is how hardy they are..and this fish looked SO healthy.

My filtration is complete with fiber, and bio-wheels and rated for up to a 75 gallon tank,,, since this tank is 55 gallons,, I assume it is well within the acceptable range,,,The tank capacity according to all the reading I have done, is approximately 24 inches of fish,,, I had, before the fish died, about 8 inches of fish in it...The substrate is medium sized gravel,,, and is light in color,, I did mention that the tank was "as is" now for about a month,, thats a lil misleading,, I have had the tank for about 4 months now,, and that's how long I've had the Gourami.... I have had a few fish deaths due to ich, but that came as the result of the rookie mistake of adding unquarentined fish, and adding too many, too fast. The last time that I have had ANY kind of problem, as far as fish health or infection goes, has been close to 7 weeks, when I lost a fish to ich. So, in short, the tank has been disease free for at least 7 weeks,, and all the fish within look strikingly healthy, and are quite at peace with each other. This loss of this Gourami is very baffling to me. There were no previous signs of any problem with any of the fish. One last thing, in my original writing I said that the morning he died, I noticed him swimming virtually vertical, changce that to "perfectly vertical", he was in obvious distress,, I just can't figure out why.