Question
I would like to install a pond for small fish in my back yard.  I seem to remember reading that Japanese Maple tree leaves were toxic to fish.  Is this correct?  Do you know of any trees I should avoid because of toxicity to fish?  

Answer
Some trees and plants are more toxic than others.  In order for a toxic tree to harm the fish, it either has to drop enough leaves/fruits/etc. into the water to raise the toxin in the water, or the fish have to eat those things.  Dwarf Japanese maples are commonly planted around koi ponds.  They only drop a lot of litter when they de-leaf in the fall.  I do not recall ever reading that maples were particularly toxic.  Here is a page listing plants potentially dangerous to fish:

http://www.bonniesplants.com/how_to/poisonous_plants_to_fish.html

I would avoid planting large trees, nut trees, fruit trees, etc. because they will drop more into the pond.  Most of the trees that can cause poison problems are nut or fruit trees.  Don't be overly worried.  I see mimosa is listed on that page as a toxic tree and yet I have one about 10 feet from my pond and haven't lost any fish from poison that I know of!  Anyway, maple trees are low in toxicity but still clean up those leaves and don't plant trees too close to the pond (for debris reasons and also the roots can puncture the liner).  Good luck!

Robyn
fishpondinfo.com