So this is a little out there. I'm not sure if anyone can answer this but I was eating some loquats (aka japanese plums) and I started getting curious if fish could eat them. I imagine in the wild fish eat all kinds of unusual fruits that fall into ponds and streams etc. I wonder what kinds of fruits would be toxic and which would be safe?

Fruit is not as common as you might think as a fish food. yes, there are species out there that eat fruit like pacu and most of the genus brycon. However, I fail to see any fish available commonly in the aquarium trade to eat fruit. Also, plums which are a type of stone fruit contain high amounts of poisons in the pit with cyanide being one of them. If you have sensitive species and a small tank, this could do much harm to your fish. I would recommend you offer a vegetable instead such as washed cucumber and lettuce. Do some research on the species you have to find out what their natural diet is. Fish base and seriously fish are very good resources to find this out. I hoped this answered your question.