Lost my dwarf gourami tonight Whats going on here

I went to feed my fish tonight and found my dwarf gourami laying on the bottom of the tank dead. I had checked on them earlier and everyone looked fine. So I was unsure what the problem could be. I did all my water tests (liquid kind) and they came out as follows:

Ammonia 0ppm

Nitrite 0ppm

Nitrate 2ppm

pH 7.8

Its a 55 gallon tank with plastic plants that was set up around the first of July. It had "cycled."

Other fish in the tank are:

3 Zebra danios

6 tiger barbs

4 angelicus loaches

I'm pretty sure it wasn't the barbs pestering the gourami. They chased him a few times when I first added them but hadn't messed with him since.

I'm really anxious to figure out the problem and solve it before I lose anymore fish. I love all my fish but I'd especially hate to lose my loaches as they were $20 a pop.

Any ideas of what the problem could be and/or what I should do to make sure I don't lose any more?

The 1st thing that comes to mind is your water is too basic. You should have a pH between 6-7 for these types of fish. Also given how basic it is. I'd suspect your water is fairly hard. All of these fish do better with a KH around 10.

I'd test your water hardness, and if it's high slowly reduce it. This can be done by gradually replacing some of your water with distilled water. Remember pure distilled water is bad as it lacks minerals, and it too soft. Once gotten your KH down then check the pH and try to get it to 7. (Hard water is harder to alter pH.) Remember however bad current conditions might be rapid change is always worse.