All my Gouramis are just sitting at the gravel!

[newhobbist]

I've noticed earlier that some of my Gouramis started to spend more time at the bottom of the tank. Now all Gouramis are spending most of the time hiding. Even Bolivian Ram, the oldest and healthiest fish in the tank, swims less and spends more time under leaves of banana plants. They do come out to eat and have good apetite, but several minutes after feeding, they hide again. Here is some water parameters:
temperature - 79
pH - 6.0
amonia - 0
nitrite - 0
nitrate - 20

These parameters have been quite stable for last 4-5 months. One of my Gouramis flashed against rocks as if it had some parasites. Besides that, all fish looks healthy. All fish have been in my tank for at least several months (no new additions).

I have no idea what would be a reason for this. pH is quite low for community tank (our tap water pH is 6.6), but I don't think that Gouramis could have bigger problems with that. What could it be?

 

[HannahJ]

They might have the beginnings of a disease. If your fish are subdued and one of them is flashing, it sounds an awful lot like the beginnings of ich, or something similar. You might consider pre-emptively raising the temperature of the tank to 87 degrees, which will kill ich in its free-swimminf stage, but some more observation might be needed before you decide on anything. Have you added any new plants to your tank lately? They can carry diseases just as fish can.

 

[Zagz]

Moved to unhealthy fish. Ich can be present even when not visible. That is not to say that is the problem. It might be you need more aeration in your tank, try adding an airstone or lowering the water level a bit to create more bubbles. Have you added anything new to the tank recently?

 

[newhobbist]

I haven't added anything new to the tank in last 6-8 weeks.
Some of my fish were flashing at one point of time or another, even a year ago.
Another thing I noticed is that two Paradise fish, my latest addition that I bought 2 months ago, sometimes go to opposite corners of the tank and stay in the corners (top level) for quite long time. They even sometimes bend their bodies and stay like that almost attached to the glass.