The descriptions all look wrong.
The tank looks more like 2ft long, 1ft wide and 1.5ft tall which makes it around 20g. This is evidenced by the length of his fish. His oto cat is most likely around 4-6cm in length. His black neon tetra probably around 4cm. This would put his discus in the range of about 8-10cm in length.
10% water change weekly. He must have some pretty awesome water quality in that tank to be able to change so little for discus.
His stocking looks like:
2 discus
2 angelfish
2 serpae tetra
1 black neon tetra
1 oto cat