Question
Hi Robyn
We moved to our new house on 9 June 2006 and as a result, we now have a natural spring pond (5m x 4m and 4ft deep).  The previous owners sadly neglected this pond for the past 5 years and, although it was stocked well with both 3 medium sized Koi and approx 50 goldfish, when we arrived the water was very green, dirty and you could hardly see the fish.  Since the 9th we have invested hundreds of pounds in 2 filter & UV systems, air blocks, plants, as well as time in looking after existing plants, such as oxygen and lily plants.  As a result, we were beginning to see the fruits of our labours and the water is now becoming nice and clear.  On 5 July we added 5 baby Koi and they seemed to have settled in happily.  But sadly over the past 2/3 weeks the fish have started to die at the rate of 4-5 Goldfish per day, plus we've lost a couple of the new baby Koi!  We've tested the water and a local aquatic shop has too, both have confirmed all is well with all tests.  We have had a hot weather spell and thought oxygen levels may be low?  So yesterday we added 400gallons of fresh river water, raising the water level by a couple of inches, but the deaths are increasing in numbers. Today we lost 11 fish!  There are no visible signs on any the fish of why this is happening.  Can you offer any steps we can take to find out why this is happening?

Regards
Julia Harvey

Answer
Congrats on your new pond and efforts to clean it up.  I'm sorry that you've lost some fish.  When the water was tested, did they test for oxygen?  The heat absolutely reduces the amount of oxygen that the water can hold.  How well do your filters aerate and move the water?  I suggest adding an aerator (something to froth the water, a bubbler, etc.).  The river water came from an actual river nearby?  Well water, spring water, and tap water all can be very low in oxygen but river water should have a good amount of oxygen if the river moves swiftly.  If the water you added was also low in oxygen, that could have made things worse.  If not, then perhaps the water chemistry of the new river water was off in some other respect such as containing pesticides or high nitrates.  You should also test the water for nitrate and phosphate if there's any chance of runoff from farms or chemical use in the area.  Do the fish gasp at the surface or just plain die?  I lost half a dozen goldfish myself last month in the 90+ degree F air when they turned off our power, only for 4 hours but I then lost all those fish over the next few weeks, probably from low oxygen.  You can add some activated carbon to the filter in case there is any chemical contamination.  But, be sure to aerate that water well in case it is oxygen or at least that is probably a factor in this heat!  Good luck!

Robyn
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