Sick tiger barb - curable?

[Rollux]

We have a sick tiger barb. For the last few days he has been sitting in the water stream exiting the pump, dorsal fin out of the water and gulping for air. He/She has now sunk to near the bottom of the tank, caught up in a plastic plant near vertical (head up), gulping slowly. He has been like this for 2 days now.
Water parameters - ammonia 0.5ppm, nitrites 20ppm, nitrates 40ppm pH 7.0 (No real change for the last 3 weeks)
PWC about 30% done twice a week.
Other fish are one other tiger, a single silver dollar and a bristlenose pleco. No live plants, 150Litre tank
We inherited this tank from a friend and are in the process of setting up another tank for QT and temporary home for us to clean this tank and start again.
Any ideas on the barb, or would euthanasing him be the best bet?

 

[HUKIT]

If those water change results are correct it won't be the last fish to suffer and die, the fish is suffering from nitrite poisoning. I'm assuming this is a new tank that is cycling still, you should be doing daily 50% water changes to keep the ammonia and nitrite levels under .5ppm at the minimum. Those are the first and most important thing to start on TODAY. Below is a article explaining all the steps, and other options. To help speed this process taking media or gravel from a existing tank and placing it into your filter will help decrease the time.

Aquarium Nitrogen Cycle and cycling. Methods for ammonia, nitrite removal.

 

[Rollux]

Have read that article before - I know the theory but despite PWC etc the levels always seem to come back and sit stable about those numbers.
I was given the whole setup from a friend as it is as he has no room in his new house. he had it for about 3 years, but had no interest in it once fish started dying 2 months later. The ones left are the ones that survived all that time
But now we are one less.
I am in the process of cycling a new tank so i can start a fresh with a planted tank and have it cycled before swapping the fish back over.