My pond is a smallish cascade affair. About 10yrs ago I kept fish
(Golden Orfe) in it. However, the local heron scoffed the lot, so many
years passed without fish. Because of this (and perhaps foolishly) I
planted a laburnum tree a few feet away.

A couple of Springs ago, a friend rescued some fish (poss. small Koi,
definitely carp) and asked if I'd keep them in my pond. I did so and
they survived very well, right through last winter despite the pond
icing over. Indeed, when we cleaned out the pond in summer, we found
lots of baby carp, so assumed they were doing well. This winter,
however, has seen several losses. Indeed, about a week ago I removed 4
dead fish floating on the surface.

Since the fish survived one hard winter, I find it hard to believe
they've succumbed to cold this year. There is a bubbling spout to
aerate the water, so that's not the problem. So now my dread thought is
that the Laburnum tree is poisoning them (due to leaf and seed fall),
even though there were no ill effects last year.

The Laburnum tree (which came pot-grown from the same friend) has been
in place several years. The circumference is 11", so about 4" diameter.
Moving it would be a huge job for me, but one I'm having to consider
all the same. I know this friend would be very upset if I lost the
tree, so chopping it down isn't an option, but it puts increased
pressure on me should I move it. I know that, ideally, I should trench
and root prune it this year, making the rootball smaller and keener to
grow away. Alas, I'm feeling the need to move it urgently, so may not
have this option.

I would be grateful for any input regarding the fish/poisoning/other
cause of demise and the chances of moving this tree successfully. The
tree was fairly big in its pot before I received it, so it may have had
a slow start in life due to a limited rootball. I'm hoping this is the
case, but fear it may have established a strong tap root and made itself
at home. Help, please!!


--
Spider
from high ground in SE London
gardening on clay