Decking of the wrong sort will kill your fish.
Many of the wood stains on the market are
fine until they get wet. Some are lethal and some of the so-called "fish
friendly" are the worst.
Use cheap decking - then buy cheap fish.
Neither will last long. I am afraid this is one product where you only get what
you pay for, does not always apply. The new tantalizing methods considered safe
by the EU are very speculative.
As you will gather I am not a decking
fan. In its place fine, over ponds never!
If the chemicals don't get
your fish, then the mechanical injuries from fish coming into contact with
decking, and the following secondary infections, make fish keeping hard work.
Just a thought - have you fish with ulcers and no matter what you use
including injections the infection just get worse? I surmise 99% it's the
decking that's poisoned your fish. It works like the Victorians poisoning their
wifes with arsenic - just a little now and then till the build up in the
tissues becomes lethal. That's just how decking poisoning works, and with the
same product - arsenic which is used in tantalization, and I am not convinced
the new method is any better.
Some of these waxes are OK untill they
get wet.
One note - put decking over your pond and there's a chance
your fish will spend more time under it than in view. Koi should be where you
can see and admire them.