blue gourami dying

Question
I had a total of 6 fish in my 55 gallon tank, with a power filter and heater set at 78 degrees.  The tank is 4 weeks into the cycle, with not sign of ammonia and nitrite, and small amount of Nitrate(5ppm) I believe.  After I started my tank with 2 fish, I introduce 2 fish per week to now a total of 6 fish(which will stay at this number), and they are silver dollar, austrian rainbow, blue gourami, yellow gourami, redtail shark, and a rosy barb, one of each fish in the tank.  
After I introduce the last two fish(blue gourami and rainbow), the blue gourami were being picked on by the barb, then couple of days later, they were fine and then the blue gourami is picking on the yellow gourami to the point the yellow gourami won't eat and died last week, now with only 5 fish in the tank, the blue gourami is acting strange, it's doing what the yellow gourami did before it died, stays at the bottom all the time hiding even no other fish are picking on it.  It won't eat, only time it comes up it's for air, I am worry it will died.  Is there a problem with it, or these fish are not compatible.  Also I have checked, there are no sign of ick or abnormal about him beside the fact his whisker is shorter and bent(I think from a fight).  Please let me know what should I do, or if this fish die, I will like to get 2 fish that will be compatible with my other fish(barb, rainbow, silver dollar, and the redtail shark).

Thanks a million.

Roland

Answer
Hi Roland,
Believe it or not,   diagnose your problem as over feeding. Why do I say this? There's no such thing as a fish "not eating" as a sign of sickness.
If you have six fish, feed them six food flakes or pellets a day total. If each fish eats a pellet or a flake a day, that is enough food. As they grow larger, you may increase their diet to 3 pellets each per day. If they grow larger than a pancake flipper, you can give them 6 pellets or flakes each per day.
If you feed more, the fish will die.
Please take my word for it. Read further elaboration at my web site. Please take these points into consideration.
http://steamboats.com/aquarium.html
Nori