Is this a new aquarium or an established?
Do you know your water parameters?
Has this gradually got worse or suddenly appeared?
It looks like an algae bloom more than likely caused by excess light and/ or nutrients.
How far away from natural light (eg. window, rooflight etc) is the aquarium? How long are the aquarium lights on for?
How big is the aquarium? How many fish and what species? Are the fish being overfed?
No as son is 10 i told him i would feed them so he didn't over feed them.I would take a sample of water to test later and then do another big water change. Remove a sample first, so we know what the parameters are before you improved conditions by the water change. Make sure you use a reliable test kit, not test strips. Test for pH, ammonia, nitrite and nitrate. If you dont want the expense of a test kit take a sample to an aquarium store, and write down the numbers. Don't accept them saying everything is "good" or whatever. Get actual numbers off them.
Is there any chance a load of food might have got into the aquarium by accident? Or any other contaminants?
Thankyou i will get a kit tomorrow if they have one in town.API Freshwater Master Testkit covers what you need to test for, is reliable enough for what you want, and is fairly easy to use. Just make sure whatever you get is a liquid drop test kit though, not test strips.
Are the fish acting OK?