Problem with my Gouramis

[Sid]

I have a dwarf flame gourami that was separated from my main tank because it had pop eye. I've been treating him for 4 days with Tetracycline and he's been getting worse. As of yesterday he was swimming erractically, hiding, and had a cloudy film over one eye. He also has dark patches on the upper parts of his body (this is new as of yesterday).

So yesterday I starting giving him MelaFix and he seems a little better today, at least he's swimming a little more normally. He still hasn't eaten in 4 days.

Now, one of my powder blue gouramis has dark patches on his body and he hasn't eaten in a couple of days. I've separated him from my main tank.

Does anyone have any idea why my gouramis are getting sick? And what else I can do. I still have one healthy powder blue gourami in the main tank and I'm worried that he may get sick too.

On the Jungle strip test kit this is my water:

Nitrate 20ppm
Nitrite 0ppm
Total Hardness 150ppm
Total Alkalinity 80ppm
pH Level 7.8

This is my tap water (treated). I've heard that gouramis can be sensitive to Hardness and pH. Could this be causing any of the problems I'm having? If so, what can I do to the water?

Any help would be greatly appreciated