my fish are dying

Question
we have just come back from a short holiday and or neighbour has been feeding the fish the fish pond has only been up and running since the middle of june we had 7 largeish fancy goldfish and one koi since we have been away we have lost all but one of the fancy goldfish and koi.
the pond has a pump filter and uv light box the remaining fish are staying near to the pump at the bottom of the pond and the pond which was clean is now covered in algae have you any ideas as to what we are doing wrong the neighbour also topped up the water level with tap water could this be the cause?
any help will be much appreciated

Answer
I'm sorry that you lost half a dozen fish.  How big is your pond?  Have you tested the pH, ammonia, and nitrite levels to be sure they're ok?  If water was added with dechlorinator, that could certainly harm the fish's gills making it hard to breathe.  Does the pond have aeration (fountain, spitter, bubbler, etc.)?  What's the water temperature?  I've lost fish myself this month because the water holds less oxygen in the extreme heat.  I have two waterfalls and added an air stone for more oxygen. What symptoms did the dead fish have?  Were their bodies in the pond, out of the pond, or missing?  If they were not in perfect condition or missing, then predators could be a factor.  I suggest aerating the pond well, add pond salt per the directions (or half as much to be sure plants are not affected), adding more plants, add some dechlorinator just in case, test the water if you haven't, and perhaps some more things when/if I know more.  Good luck!

Robyn
fishpondinfo.com