What Is Drop-Eye, and How Can You Prevent It?
To start off, I want to state that drop-eye is not a disease because it can't be spread or caught. Drop-eye is the condition that causes an arowanas eye(s) to look down. Some arowanas get very light DE, e.g. the eye is slightly angled down, while there are some very extreme cases in which the eye looks as if it will fall out at any given second.
There is currently nothing that can prevent or cure DE. The only "cure" is to put your arowana in a huge pond, but this procedure has not been proven and may not work considering that wild arowanas have DE too. It has also been said that floating ping pong balls in your aquarium will solve DE as your arowana will look up at them. However, your arowana will be interested in the balls for all of 10 seconds and will then go back to doing what it always does; it might even be hazardous if you have a big arowana that can swallow the ping pong ball. In short, there are no proven cures for DE as of now.
Which Arowanas Are Most Prone to Drop-Eye?
Silver Arowana: The silver arowana is the first arowana that people think of when DE is mentioned. There has yet to be a picture of a silver arowana without DE over 20", and DE typically starts developing after the arowana hits 16".
Jardini & Leichardti Arowanas: Jardinis and Leichardtis are both Australian arowanas; they are not the same species but will be grouped together for this post. Jars & Leis do not develop DE as often as silver arowanas, but the majority of them do after hitting 16"-18".
Asian Arowana: The Asian arowana has a very low chance of getting DE. DE typically only occurs in bigger Asian arowanas (24" or larger), and is rarely, if ever, found in a smaller Asian arowana.
African Arowana: African arowanas cannot or will rarely ever get DE. There has yet to be a photograph of an African arowana with DE.
Black Arowanas: Like the African arowana, the black arowana cannot or will rarely ever get DE. To my knowledge, there are few, if any, photographs of black arowanas with DE.
What Causes DE?
So here's the important question: If there are no cures for DE, then what causes it? There is no definite proof as of now to answer this question as we still have no idea what causes DE, but we do have many theories.
Below is a list of the most common myths & beliefs on how DE is formed. There are probably many more beliefs & myths, but there are far too many of them, thus only the most common ones will be listed. I will refer to each belief by number after listing them.
Beliefs:
DE is caused by excess fat.
DE is caused by head trauma.*
DE is the result of tank reflection.
DE is caused by strong light.
DE is the result of the arowana looking down.
DE is caused by genetics.
Meaning:
DE is caused by an arowana eating too many fatty foods, e.g. feeder goldfish. As a result, the fat builds up behind the eye and pushes it down thereby causing DE.
DE is caused by an arowana hitting the sides of the aquarium too hard. The trauma and/or shock from the impact loosens the eye from its socket, and the arowana develops DE as a result.
DE is caused by the aquarium reflecting light. The light reflects into the arowana's eyes and causes DE.
DE is caused by strong lights. The arowana is bothered by the light thus it has to look in a different direction, so it looks down and DE develops.
DE is caused by an arowana looking down. Regardless of the cause (light, food, etc.), when the arowana looks down, it starts to get used to this behavior and eventually this behavior becomes permanent thus it develops DE.
DE is caused by genetics.
What Doesn't Makes Sense?
If DE is caused by fatty foods, then why do only some arowanas get it? Why can't we prevent DE by feeding lean foods if it is caused by excess fat buildup? Why don't black arowanas or African arowanas get DE even if you feed them feeders?
If DE is caused by head trauma, then why would Asian arowanas be less prone to developing it than silver arowanas? Why would they have DE in the wild if there is really nothing for them to ram their head into in the wild?
If DE is caused by light reflections going into the eye, then why can't it be cured by simply turning off all the lights and not letting any light get to the arowana?
If DE is caused by the arowana looking in a different direction due to lights, then why can't we just cure it by turn off all lights and wrapping the tank in black?
If an arowana gets DE by looking down, then why can't we just take away all factors that make it look down?
If DE is caused by genetics, what makes most, if not all, silver arowanas get DE while only a selected number of Asian arowanas able to get DE?
What Makes Sense?
When you think about it, when humans gain weight, they get bigger and bulkier. If an arowana worked the same way, then there could be fat build up behind the eyes thus pushing them out.
As popularized by movies, someone gets hit in the back of the head, and their eyes come pop out, so this may also apply with an arowana when the arowana rams into the tank glass. This would also explain why silver arowanas and Australian arowanas are more prone to developing DE than Asian arowanas. Consider the fact that silver arowanas and Australian arowanas are skittish arowanas and will dart when they get scared (which is very often) and is not surprising if they hit the side of the aquarium when they dart; Asian arowanas are more calm and do not get scared as easily thus this behavioral difference explains why Asian arowanas are less prone to DE. African arowanas are the calmest out of the arowanas which means that they will rarely dart into the side of the aquarium thus they rarely ever have head trauma.
Lights are harmful, especially to the naked eye. If human eyes can be hurt by light, the arowana eyes might also be the same.
It is a basic reaction that you either block your eyes from it or move your eyes elsewhere when something is shot at your eye. Arowanas have nothing that can shield their eyes, so they would need to look elsewhere.
Humans have habits and so do fish. If we start to slouch for a long period of time, then we will become accustomed to it; the same can be said for an arowana looking down.
Genetics would explain why silver arowanas are very prone to DE while black arowanas are virtually immune to it even though they are closely related. This would also prove why you cannot prevent or cure DE.
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