WHAT! 31 degrees c is about 87 degrees F!!! LOWER THE TEMP FIRST! you should be around 80F max!!!! Have it around 78 degrees F which is about 26 degrees C. I know hotter water leads to a faster metabolism but if it isnt a asian aro you will have negitive side affects and will shorten your aros life span, please lower the temp to around 26C.
As for the swimming sideways, do you have lots of aeration? If so cut back on the aeration because its a swim balder problem. Cutting back on it will solve it but it may take a while.
For the drop eye, you maybe feeding to much. The drop eye causes are the following 1. Over feeding/Fatty Foods, 2. Genetics, 3. Reflections. Look at the top 3 reasons and read the fixes I post.
1. This is the most common, everyone wants a bigger fish which leads to feeding more. If you do feed to much limit back feeding. This theory which many have proven is that behind the eye is a fat sack which stores fat. When you feed to much this fat "sack" gets bigger pushing the eye down. Starving/fasting the aro will eliminate the drop eye.
2. As aros became more popular there are many bad breeders who inbred aros which cause drop eye. If this is the case cross your fingers that it will get better. There is no fix for genetics. Many people have said putting them in ponds will fix it within two weeks. So if you have a pond throw it in there! But if you dont, dont worry too much about it. Maybe it will go away on its own.
3. If it is reflections its a easy fix. Just get back grounds and maybe back grounds for the sides of the tank as well making sure you have the bottom covered too. As in rocks paint etc. But do it fast there is no time frame but refection can harm the fish permanently.
And to answer your question about why do they have drop eye if you only feed them pellets is simple. You're pellets have a HIGH HIGH fat contain. Try to switch one of their meals a day with market prawns which are simply uncooked market shrimp. Market shrimp is great for a stable diet.
Good luck.