Time of year matters!
I have cut elms and cottonwoods in the spring and they weep fluid out of the cut stump until the wood seals off by itself.
I do almost all of my tree clearing and treatments in the fall after the first hard frost - when the sap is flowing back downwards into the roots.
Also, you must treat the tree immediately after cutting. Trees are quite efficient at sealing off any "damaged" areas. Get the herbicide on immediately and let the tree draw it into the roots.
My advice, cut and treat one or two trees with Tordon right now. Check your results next spring when the willows go back to full growth. I can't believe your fish would have any risk from that.
