Green Water In Hospital Tank

[umberk]

So I have a hospital tank that is 29 gallons with a divider. No gravel, sparse decorations, 2 30 gallon filters, temp around 82 degrees Fahrenheit.
Nitrates below 20ppm, Nitrites are 0, Ammonia is 0, pH around 7.2. The light is on for about 9-12hrs. No idea how to test Phosphates as there is no testing kit in my local stores for it.

I have an issue with green water occuring. Sometimes it takes a couple weeks, sometimes a month or so. Im not sure why its occuring, I will break down the aquarium when I have no need for it(no reason to run a tank for several months with nothings inside), I also partially started doing it because the green water was occuring thinking if it was cleaned out that it wouldnt reappear. I do 10% water changes 3-4 times a week. Right now the tank has two 3 inch oscars on one side and a 4 inch koi on the other.

None of my other tanks, planted or unplanted, get green water even if the lights are on for the same amount of time or more.

Any solutions besides using a UV sterilizer?

 

[fluttermoth]

If you're not growing plants, there's actually no need for lights at all; they're just for our convenience for viewing; you certainly don't need them on for 12 hours, so try cutting back on the light and see if that works.

 

[James9114]

koi as in koi carp? if so these are cold water fish

 

[umberk]

koi as in koi carp? if so these are cold water fish

I know they are cold water, ick was the issue. They are only in it until its gone. Warm water and medication makes ick go away faster by speeding up the parasites life cycle.