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I found a bunch of caterpillars in one of the trees in our house. I took one and dropped it into my fish tank, thinking that all my fish would love to eat this Live Food. But, I was surprised when I saw that none of the fish showed any interest in eating it. Even, most of them just passed by it, as if they did not notice that. After a few minutes, when I checked it again, I found the 'dead' caterpillar, without any damage to its body... I was thinking about, what could be the reason for my fish behaviour... Do the fish also have a fear towards 'Caterpillars' as we humans do..??!

Some caterpillars are poisonous and most caterpillars taste very bad to avoid being eaten by predators in nature
Be specially careful with the colored ones!

I think the fish don't considering caterpillar as food. Once they recognise it as food they will eat them. Please also note that some of them are also have poison.

Caterpillar in the tank has been dead due to non presence of air. They can not breath in the water.

each caterpillar has distinct defense mechanism,to protect themselves, which are usually not visible to our eyes. they are only edible if you have removed the internal contents of the body, by pressing it from one end. only few fishes carry the immunity to handle the immediate discharge of caterpillar that they release on threat. In wild usually all fishes do not consume everything, they usually throw off the intestine while others consume it without any issue. you can take the caterpillars and dry them. then they can be easily used as food source for domesticated fishes.