where is your tank situated?  Is it near a window?
The green murky water sounds like an algae bloom?

For how long do you keep the light on during the day?
10-12 hours should be just fine, but longer sometimes may cause algae blooms.
How big is your tank, and how many fish?

oh sorry, i have 4 fantail goldfish and an alge eater

Well i was advised wrong then when i set up my tank 2 years ago,  a 120 litre tank is more than enough for 4 fish!!

Thanks for your advise, but i am really getting alot of people telling me different things, when i first enquired about my tank and the size i wanted the guy in the store ( an experienced fish keeper) told me that 6 fish would be the max  in a 120lt tank!!! when i set up my tank 2 years ago i had 2 fish then a few months later got 2 more and to this day theses fish are going strong!

Hmm so what makes him more right than us? He may be an expert on fish but not necessarily goldfish. I was just treating a fish for a disease and the the guy at the fish store who is also an expert didn't tell me a VITAL thing about the medication I was about to use. People in fish stores aren't generally that great, they are about selling and more often know more about tropicals and less about goldies.

Go to any forum on the net where people are dedicated to goldfish and they will tell you exactly the same thing, 40 litres per goldfish!! Now of course your fish isn't going to drop dead because you are a bit short in your tank, but the problem could be caused by this. I had a goldfish in a 22L tank and no matter what I did, the nitrates just kept building and building till it really started to get to the fish.

Seriously we know what we are talking about It can get confusing but I promise 40l's per goldfish is right.

Hi Aquaone and welcome to the forum.

Basically it is correct that one should provide 10 gal ( about 40l ) per goldfish. They really are "messy" fish, eat and poo a lot and grow fast. Everybody who replied  to you is right.

At the same time I say the tank isn't FAR to small, but it is pushing a bit. Like mine, we've got 5 Comets in 150l tank ( 3ft ) it is pushing too,  . But at the same time if you provide really good filtration and aeration one can manage. I am aware that I will need to upgrade eventually. It is very, very important to keep the water quality pristine, 0 ammonia and nitrites and nitrates 20ppm or less. If they go over 40ppm please do a 50% waterchange, using conditioner and beneficial bacteria. Apart from this, waterchanges should be done on a weekly basis anyway

Most of us on here have had the experience of being wrongly advised by " experienced " fish keepers. You'll learn by your own experience too.

But at this stage we must handle the given situation: Please give us the proper information about your equipment too ( kind of filter, flow rate, feeding habits, some pics.