I second (or third, or fouth) the "add more filtration" opinion. I suggest more water changes than 1 a week. If you do 1 per week, make it 50%, but I favor 2-3 per week at 20-30%.
In my 55 gallon, I have:
1 Senegal bichir (7")
1 Rope fish (10")
1 Endlicheri bichir (5")
1 Delhzi bichir (5")
1 Palmas bichir (4.5")
8 Buenos Aires tetras (2-3")
1 Frontosa chichlid (2.5")
2 Bristlenose plecs (4")
1 Clown plec (3")
I run a fluval 203 on it and a penguin 550 powerhead. It is heavily planted. I do water changes 2-3 times per week, 20-30% at a time, and my nitrates stay at 5-10ppm.