Why is my goldfish's tank green a few days after i have cleaned it?

You have an algie situation. Remove the fish. Boil all the rocks & plants to kill off everything you can. Scrub the tank with as warm or water as you safely can. Refill it with clean water & an algie destroying solution ( you can find at most pet stores or department stores in the pet section ). Let it sit out of the direct sunlight for a couple of days before reintroducing the fish. If you don't have an algie eater or some other method of removing the algie I would not put it back in the window as the sunlight is encouraging the algies growth.

It's an algae bloom. keep your tank away from direct sunlight and keep your tank light off for a while, even blacking out your tank for 48 hours might help. keep up with weekly 15%-20% water changes and it will gradually improve. Do not add any "algae eaters", goldfish do not play well with others. algae eaters are also high waste producers and may overwhelm your filter. as well as the vast majority of algae eaters requiring tropical temperatures. (and the fact some suckermouth fish will munch on fish's slimecoats rather than algae!)

so it means not the goldfish which turning green, am i right? well, i am not sure what is the caused of this matter, perhaps there is still some algae which still exist which make your tank turn to green. does it always happened whenever you clean your goldfish's tank?

My theory is, maybe your water contain the seeds of green plant, may be chlorella or other small algae, as you leave it in a couple a day, it grow. And the sunlight help them grow.

Do not clean the gravel and the ornaments in the tank or you will start the cycling all over again. Cycling means that the fish are giving off ammonia as a waste product and ammonia is harmful to fish. There is a bacteria that will start growing in your tank that will break down the ammonia into nitrite which is also harmful to fish. There is a second type of bacteria that will break the nitrite into nitrate which is not too harmful to fish. So eve3ythime that you clean the tank you kill these good bacteria and the ammonia will built up and stress or even kill the fish. As far as the green water , cover the tank completely for several day since you do not have any more live plants in there. Also yo probably have too many goldfish in a too small tank, just because they are not bumping into each other does not mean that there is enough room for them. Fancy goldfish need at least 20 gallons for each fish and common goldfish (the ones with only one tail instead of two tail fins) need at least 100 gallons to properly grow and swim. Don't worry, when I was young I did the same thing and wondered why the fish were dying.

Move the tank away from the window and put the plants back in if they are living ones. Also make sure you are not over-feeding your fish, or cleaning out the tank too often. You have Algae and it's a very common problem. Live plants will take up the nutrients in the water making it difficult for the Algae to survive, though normally gold fish are likely to feed on the plants and can dig them up. The sunlight encourages the growth of the Algae too and also heats up the water and it's best to site a tank somewhere that sunlight can't directly hit the glass. Good luck getting rid of the problem!

You just answered your own question, graceandowen. The sunlight is causing algae to grow rather quickly. Move the tank to an inside area away from sunlight and you should have no more problems. I have a gallon goldfish bowl that I change maybe once a month but only cause it starts getting cloudy then. My only goldfish is happy in it. Now if I can just remember to feed him couple times a day. lol