freshwater tank problem

Question
Hello,
I have a 55gl freshwater tank. Set it up about 6 months ago. Two weeks after I introduced fish in to the tank I upgraded the filter to the Emperor 400. I went through a lot with this tank. Fish died, bad odor, cloudy water, fish disease, fish discoloration, more cloudy water, more fish died.....(I had 29gl tank before, so I introduced my fish to the new tank).
Anyway, for the last 3 months and a lot of work I see some improvements, the water is crystal clear now, but.........
These are the parameters:

55 gl freshwater tank
Filter: Emperor 400 bio-wheel
3 kissing gouramies (2in each)
2 black ruby barbs (1in each)
1 tiger barb (1,5in)
1 pleco (5in)
2 live plants
Temp - 81F
Ammonia - 2.0
Nitrite - 1.0
Nitrate - 20
Ph - 6.0 (or lower)
High range - 7.4 (or lower)

Additional information:
Weekly water changes
No overfeeding (small pinch of flakes and 2 algae discs for pleco once a day)

Please advise me what should I do? I've done a lot of research, but did not find info needed.

Thank you.

P.S.
Maybe I did not fully explain.
My concern is the high readings. And Maybe I am wrong, but I don't think I overfeed. I only feed a really small pinch of flakes. Since pleco do not eat flakes, but gouramies enjoy to munch on the algae disks, my pleco only gets a tiny bit of food to himself. So if I cut on the disks he'll go hungry.
Best regards,
Elaine


Answer
I would feed them twice per day what they will eat in 3 minutes of time.  I would also do a 30% water change and clean the gravel.  Somewhere in your tank something is rotting... the decomposition gives off ammonia and that begins the nitrogen cycle.  Find the decomposing matter and the cycle will be broken.  If you need more information on the nitrogen cycle... let me know.. dave