Many dead big goldfish

Question
Hi, we recently set up a pond and after a week of the 15,000 litres of water being put in, we imported some fresh goldies (16 young ones) from a reputable aquaria, these ALL are alive and doing well.

We then adopted about 40 large (6inch) fish from a lady who had to empty her pond as she was going into a nursing home. they were healthy and happy in a pond about 1/5th the volume of ours. These fish happily swam around all through winter with ice etc no probs.

Over the last 3 days I have had one extremeley healthy looking specimin float, then another the next day at bottom of shallow end, then this morning three more floating.

One of the floaters today had white eyes and some yucky slime.

Water passed all ph no2 no3 and the other test from the expensive test kit.

I may have been over feeding them a little but not .

I have a 2500 l/h pump running into a trough with tons of algae on side of pond liner overflowing back into pond.

The big fish are noticeably sluggish.

The first dead one, had a large anus/rectal opening and was quite fat, when sqeezed (quite hard) white/milky ooze came from this orifice.
I have not put in any broad spectrum anythings though I am really tempted to do so.

I would greatly appreciate your help.

Answer
I'm sorry about your lost fish.  What was your actual results for the pH, nitrite, and nitrate?  Did you also test ammonia?  What about hardness?

The fish with the large vent that was fat probably was a female full of eggs.  Females with eggs may die after being beat up in spawning but are also more susceptible to water quality problems and various illnesses than other fish due to the extra burden of the eggs.

The white eyes and slime may have occurred after death if you didn't find the fish for a while.  If it indeed died like that, then it's probably either a bacterial problem or a result of a water quality issue.  Toxins including chlorine, ammonia, and nitrite can cause fish to slough off their slime coats and get cloudy eyes.  Your expensive test kit I'm sure doesn't test for everything such as pesticides.  

I suggest doing a water change, maybe 20% if you can.  Add dechlorinator if you have city water.  Put in 0.05% pond salt.  That helps with many things.  Put some fresh activated carbon in a mesh bag in the filter or near moving water.  Add some MelaFix (all natural antibiotic) by Aquarium Pharmaceutials.  You can try the antibiotics but monitor the ammonia and nitrite as sometimes antibiotics kill off the good filter bacteria as well as bad bacteria.

I suggest trying to find a local pond club. They may be able to send someone out to access your pond, put scrapings of the fish under a microscope to try to diagnose things, etc.

I hope that helped a little.  Good luck!