Gourami Darting Around And Swimming In Circles

Hi, I've had my three gourami, 2 golden and 1 blue, for about a year now. Recently one of them has been darting around the tank, hitting itself on the gravel in the bottom of the tank and then jumping out of the water and hitting the lid. After doing that for a few seconds it will drift around the tank, sometimes floating upside down, and then fold itself almost in half and swim in tight circles like that with it's gills open really wide. But then after about 30 seconds are up the fish goes completely back to normal and acts like nothing happened. It is only happening to one of them and it only started last week. I saw it happen once last week, and then once again for the past two nights, but now it's happening a couple times a day and I don't know what's going on. I'll try and post a picture of it happening when I catch it next time, but for now here's a picture of the fish normally, it's the fish in the upper left.

Skipped over the topic saying that I should post this first so I thought I would add it now.
 
Tank size: 20 gal long
pH: 7.6
ammonia: 0
nitrite: 0
nitrate: 20

tank temp: 75

Fish Symptoms (include full description including lesion, color, location, fish behavior): see above

Volume and Frequency of water changes: ~40% water change every 2 weeks

Chemical Additives or Media in your tank: none

Tank inhabitants: 2 golden gourami, 1 blue gourami, 1 algae eater

Recent additions to your tank (living or decoration): a new aquarium rock about 2 weeks ago

Exposure to chemicals: none

It sounds like a bacterial problem that has affected it, these sorts of problems you cannot tell with test kits. Try doing a daily water change, approx 10% for a week, give the gravel a good vaccing too, see if this helps the fish first. You could also try some Vitazin, you soak food in it, this will help boost the fishes immune system and add some vitamins.