Besides what others have asked, more of my musings....

When I say water quality is good I don't have actual numbers. When I tested,
the kits had a colour chart to go on which indicated that the nitrate,
nitrite and ammonia levels were in the acceptable band.

That's too bad (no numbers), as the only acceptable measure of nitrite &
ammonia is zero and sometimes those color charts will fudge a bit. As in, a
little is still acceptable to them.

the ph is 6.5.

That's kind of low, if I remember right you said your pond has been in
operation a number of years, and I'm wondering if maybe you might (have)
been overcrowded and if not watching the KH perhaps you've had a pH crash
(usually though this shows up with readings of ammonia). The fray
fins/tails though, usually signal ammonia. Perhaps because you've lost a
few, the test didn't register at the time you took it?

The product I've used with success is KoiZyme, also known as Lymnozyme.
This product gets rid of the bad bacteria by starving it, it doesn't not
hurt the bacteria in the filter.

If your pet shops test water, I'd take a sample to them and find out the
numbers and have a KH test done. ~ jan
~Power to the Porg, Flow On!~