shimmy or shakes. . .

[nolash]

i have a problem with only four fish in my tank that will just stay in the same corner and shimmy but only when i have the light on. now i have lots of plants and decor to dim the light but they still do it. when the light is out they swim fine with the other fish. it is 2 black neons and two rosbora harlequins that do this. what could be causing this? here is the breakdown on my tank:
1 albino cory
2 panda cory's
3 cardinal tetras
4 blue neon tetras
2 harlequin rosboras
2 black neons

it is a 20gal long tank with an aquaclear filter and an undergravel filter system with 2 powerheads. all of my water parameters check okay:
0 ammonia
0 nitrite
20-40 nitrate
ph is at 7.0

i have tried 4 different types of bulbs as well, a 10000k, 6500k, 50/50, and an actinic blue but to no avail.

any help would greatly be appreciated