Hi Boggy,
Do you have any problems with the red hooks being jumpy? Mine are in a well planted aquarim and the two largest are still nervous, but yours in the 5x2 tank has a small amount of planting and what appears to be quite a bright lighting tube. I have 4 other dollars in the group, but they are not as big.
Could it be that at 6" they are getting too big for the tank? The tank is 48"x12"x15" and filtered by a fluval 3 and and old IPF3 from the old setup. The lighting is from 1 flora glow bulb that is almost the full length of the tank.