is fear a symptom?

Question
I have an established tank (almost 1 year).  2 platys, 4 diamond tetras, 2 juli cories, 1 danio in a 20 gallon tank.  i do water changes regularly and test the water weekly. All the fish get along, no bullying or fighting at all.  In the past week i've noticed that my 4 diamond tetras are acting differently.  They swim and hide when it's feeding time. Usually they're the most aggressive eaters.  When they used to see me approach the tank they'd start pecking the surface.  Now they stay low or actually hide in the plants.  They just don't seem as active all around either.  Is this a symptom of something?

Not sure if it's related but i lost a danio and a juli cory right around the time they started acting strange.  hope you can help!

Answer
Dear Emily,
Have you changed anything different in their aquarium that could have caused the strange behavior? Such as re-arranging or adding new tank decor or plants?

One of the biggest things I would consider is the water quality. I noticed you mentioned losing a danio and a Juli cory around the time the diamond tetras started acting strange. Maybe there is a water quality issue or even an illness causing the loss of the danio and cory and now it may be affecting your Diamond tetras.

Be sure if you can, check your water quality with an aquarium test kit-- Make sure your ammonia and nitrite are always-0 and nitrate 20 or less. As always, when in doubt, do an immediately 50% water change being sure to make the replacement water equal in temperature to that of your aquarium and be sure to dechlorinate it with a good water conditioner.

I would repeat these 50% water changes and gravel vacummings for several days and watch for improvement on your diamond tetras. It's best to do water changes anytime your fish are acting odd or are off-color as water quality issues are often greatly to blame.

I hope this helps!
Best wishes and Happy fishkeeping!
Karen~