About Drop Eye
Drop Eye is mainly seen amongst the Arowana family, including Silver Arowana and Arapaima. It is so called as the eye will appear to bulge out and point downwards. There is no cure for this condition, it is irreversible.
It seems this is caused by several contributing factors including bad breeding and a fatty diet of fatty foods such as Goldfish. That said, having heeded all the common advice - ie fed only (low fat) floating foods, over a gravelled tank bottom, with no distracting tankmates - some aquarists have seen it suddenly occur one eye at a time when their arowanas crashed into the tank.
Do not feed these fish a fatty diet, make sure their food is as close to what they'd eat in the wild as possible and very lean.
Arowanas are deft leaping hunters that launch with enough force to climb to several times their body length, so let them see the glass lid is there - security glass with inbuilt wire mesh or even just a grid drawn in marker pen - or risk collision induced drop eye.
Know the source of your fish and do not buy cheap or unhealthy looking specimens.
Keep the fish in overly adequate surroundings without very bright lights and with a matt substrate such as sand or river rocks - not bare bottomed. Make sure the tank is very well filtered and mature. It's said the reflection from the bottom of a bare bottomed tank could be a contributing factor to drop eye in Arowana.
- Arowana Club - Article on Drop Eye