Question
I have a 12x15x5ft deep pond with a rubber liner. the water is pumped from the pond thru 2 large 5foam filters with a uv light in the filters.The wattage is 25 watts ea. I have approx. 40 large goldfish and 10 large koi...some are 3 years old. the water ph tests right on as does the ammonia and salt level.
 Since the end of last year and all of this season, we have been plagued with floating green algae. We tried in july to empty the pond down to 2 ft and took out 20 goldfish. We refilled with well water and readded pond start.
 we have at least 5 times with time in between added pond clear chemicals. Nothing works. the floating plants we always add didnt grow this year( the first time for that)so I know that is a factor.Ive tried cleaning the foams,backwashing regularly. No matter what we do...nothing works.
I know it is the end of the season but if I could find out what the problem is, I can at least be prepared to fix it next spring. The fish are barely eating now and this is the 3rd winter they will be going thru. They are healthy and show no problems. We would like to be able to see them other than when they come up to eat.
 I might add, i also vacuum the pond and net it in the fall to keep the leaves at a minumum.I also put on a larger pump this year hoping to increase the flow thru the filters. If you require more infor, please contact me. thankyou  ps..in case you don t know, i live in northern ontario

Answer
Hi Wendy,
Sorry for the delay.
The pond is of A nice size and suited for koi and goldfish.
The amount of fish is somewhat high, as normally we do not go over 1 fish for each 1000 litres.
However not the reason as they are goldfish and they tend to eat less.
So there is no problem.
As I may think from A distance... You could need A more mature filter?
I do not know if you also clean the foam as this is not ideal because you lose bacteria.
Best way to have A filter is to have A mechanical filter to remove all visual debris.
In this you can use, sieve, vortex, etc and also foal witch you clean often.
Then you have A biological filter with A material that can contain bacteria.
For example foam, Japanese mats, Kaldness, moving bed, beadfilters,...
After this you can add A UV and you need to take op to 3 Watt per 1000 liters.
Lamps need to be changed every year.
This filter needs to run all year, the water go's best completely trough within the 3 hours.
Wintertime you can put off the UV.
In springtime you need to start your UV and add bacteria to your filter.
During the season best way to go is to change approximate 10% water weekly.
This should provide you enough fresh water and controlled filtering to enjoy your pond.
Hope this helps.

Warm regards,
Raf