by hand go for this YouTube - arowana-3

Dropeye

This is not a disease, it is most probably cause by change of nature to captive environement which encourage the fish to look side and down rather then up. The 2nd most popular hypothesis is overfeeding. Genetic is the biggest factor in this problem.

Possible Cause
Downward and lateral gaze -Transfer to pond or cover all sides of tank
Overfeeding -Reduce feeding, increase excercise(eg introducing slow current, larger tank)

Prevention is better then cure. Get a low tank. Avoid overfeeding, use floating food if possible. But most importantly do not buy the fish is you suspect dropeye.

There are other propose treatments for this problem. They included, adding ping pong ball and red flashing light into and above the tank respectively, putting a tank of cricket above the tank, pushing the eye back manually, operation, etc. I do not recommand the last three methods as they have high mortality rate.