Ponds and Water Gardens - pond turn green after a week

my pond turned green. I changed the water and it looked clear again but 5 days later it turned back green . I did it again but still no help.I used 4 layers of filter but i still doesn't help. Any suggestion please.

i took about 70% of the water out and refill with tap water everytime. I don't want to take all the water out caused that might kill all the fish. thank.

Need a UV light.

What type of filter are you using?

i just buy 4 thick white filters at the store and stack them up on top of each other

Ok, so you're using mechanical filtration. Given this method, you won't be able to get around green water, which is algae bloom.
You're going to need to spend a little money to achieve a clean pond. Here's some suggestions that will render you a chrystal clear pond:
1. Change your filtration from mechanical to bio.
A biologiocal filter, in layman terms, breeds good bacteria that eat bad bacteria. This type of filtration takes a bit of time to get started as you need a few weeks of pond water flowing through it to begin to grow the good bacteria. As well, you can purchase good bacteria to add to it for a jump start. One other thing, these filters also incorporate mechanincal filtration as well.
2. As "airman.1994" stated, you also need an ultraviolet clarifier. Even as good as the bio filters are, they still are not the end all to clear water. To ensure water clarity, get a UV clarifier and your water will be beautiful.
3. When sizing you pump, be sure your pump has enough volume to turn over your pond once an hour. In other words, if your pond is 500 gallons then buy a minimum of 500gph (gallons per hour) pump.
4. If you don't already have a water fall, add one or even a fountain to help oxygenate the water for a healthier pond.

Hope this helps.

Joe

how can i turned my mechanical into biological filter? what biological filter consist off? If i remember right, looking at it a while back, biological filter has a thick filter with three or four layers and biological balls at the bottom. Can i just add biological balls at the bottom of my existing filter to nurture good bacteria?

I can't answer that. You might contact the manufacturer to see if it's possible.
One note: good bio filters introduce turbulence into the water as it enters the filter-- this encourages bacterial growth. This would be one consideration. I might suggest you sell yours and buy new to save you a lot of hassle without the assurance it will work.
Or, if you're trying to save money, just go get a UV clarifier, hook it up to your existing setup and see if it gives you the results you want.

Joe

You say "after one week". Is this a new pond? When a pond/aquarium is first establishing it will go through an algae spell, until all the biological ecosystem is well established and that takes time (wont find it in your tap water, that’s for sure). If you want to spent a lot of money you can go buy bacteria in a bottle, but it only speeds up the natural process and can be quite costly. keep doing water changes to keep the bloom to a min if that’s what you prefer. it may take a few weeks but it will clear, as long as it’s a healthy pond! UV lighting is great too. How much direct sunlight do you get? bio balls are excellent 'homes' for the necessary bacteria needed to maintain a healthy environment. Lava rocks will also house them just as well. A lot of home made filters utilize lava rocks successfully. oxygenation is also key, make sure the oxygenated water can get to the bottom of the pond for bacteria growth, it’s the bacteria that eats up the sludge at the bottom. as mentioned earlier, waterfalls or even fountains can offer good oxygenation. Do you have other plants in your pond yet?

Good luck with your algae, just remember, it’s a normal process and will pass as long as your doing maintenance. And yes, too many water changes aren't good for fish. Carp of course are way more picky than goldfish. Goldfish will tolerate almost any environment.