Disease problem

Hello, I'd be very grateful for any advice:
I have lost nearly all my fish (25 or so) to a disease - most have
large open sores, but some appear to have no visible symptoms. I had a
variety of fish, all the koi and goldfish have died as well as a couple
of shubunkins although I have 4 orfe and they appear unaffected.
The nitrite level is extremely low, the amonia almost as low but the pH
is maybe a bit too low: 6.8. The water was a little cloudy but I've done
a partial water change and fitted a larger filter (I have UV too) and
it's looking better. I've treated the water several times with
anti-ulcer / fungus and once for anti-parasite, I've also used a tonic.
The remaining fish seem a little lethargic and are not that hungry, but
are surviving. This problem has persisted for several weeks now and am
out of ideas, your help would be greatly appreciated.
P.S. I've also treated the water with an anti blanket weed formula (non
chemical). I also have irises, bullrushes, a grass and a weed growing in
it.
My friend tells me to up the pH with sodium hydroxide he can get from
work, is this advisable?
Thanks a lot.


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marmot