Oh man you are in for trouble!. The tank is a little warm for goldfish, you don't need a heater for them because they are coldwater fish not tropicals. Somewhere between 62F-72F is good. More important is that the cloudiness (more then likely is milky white) is a bacterial bloom. The bacteria that grows is necessary but there is a imbalance of anaerobic and aerobic, not uncommon for new set ups but the problem is you are grossly over stocked, even for an established aquarium. I would start doing large 80% water changes and take back some of these fish to the store that sold them to you. They should give you a full refund for the 8 you need to return. If they did thier job and advised you like reputable stores should, they new full well that 10 goldfish (assuming they are comets) in a 20G tank is disaterous. Anyway the bacterial bloom is caused by an ammonia spike or the 1st stage of the "cycle", instead of getting involved with the detail check the link for how to start off right.
My advise is do a large water change 80% or more until you return at least 7 of the fish. READ the article in the link. If you don't return them or get a very large tank or 2-3 55G tanks the fish will die or get diseased/parasites. Any help I can offer let me know. This is not your fault but iresponcible sales clerks.

Here is a link that might be useful: FAQ from the KRIB

Thank You for the response. I will move the other fish to a different tank. Also how can I control the water temperature, I don't have a heater. That is how warm the water natually is.

If water is that warm then the house must be slightly warmer. A few things that can help though.
1) Position the tank so it isn't in direct sunlight (windows) or near a heat source.
2)If you have lighting make sure it is fluorescent and use blocks of wood or something similar to raise the fixture an inch or more over the tank cover. This to let air circulate. Even fluorescent lights can raise water temp 5-10* in the summer. If you have incandescent bulbs, go to Walmart and but compact fluorescent replacements. Incandescent raise the temp up to 5X or more of fluorescents.
3)Get a small fan like the ones that clip on the end of a table (Walmart also has these about $5-8)or an old computer cooling fan. Blow this across the surface of the water and the lighting.
4)If you have AC put it on. Water temp is usually 5* or so cooler then room temp.
5) Only use lighting when you are veiwing the fish.

Lighting is usually the number one culprit for hot water temps in the summer. Direct sunlight being 2nd. In salt water reef tanks with intense lighting people use chillers. They are very expensive but effective. In an extreme emergency do a PWC to lower the water temp. Some use ice cubes (made with dechlorinated water) I really don't like that idea but in a pinch......
Either way your water temp is on the warm side for goldfish and "may" cause stress if at that temp for extended periods according to the experts. Another reason to lower the temp. is it makes the ammonia less toxic. You have probably read that in the article I linked you with.

What type of goldfish are you talking about? Run of the mill goldfish or fancy type goldfish (lionhead, ryunkin, oranda, pearl)? Fancy goldfish prefer warmer water and are not cold-water fish.

You have way too many goldfish for that tank. If you have common goldfish, they can grow upwards to a foot long in ideal conditions. Fancy goldfish can grow to be quite large as well.

The gold fish we are talking about are comets since it is my first time raising fishes. Thank You for the advice woeisme and yes I did read the artical you sent me.

Comets are really pond fish - and unless you have a really large tank available, not suitable for indoor aquariums.

I agree that your temperatures are too high, and you should definitely try the things Woe suggested. But I also think they may be fine since goldfish survive summers in ponds here all the time. I'm only saying that to help put your mind to some ease, not suggesting you be lax about keeping the water cool.

If you're having any trouble finding space for the other fish, they sell 30g+ rubbermaid bins at Wal-Mart for much cheaper than a glass aquarium. They make for nice indoor ponds on a budget. Larger water volume is also easier to keep cool.

comets outdoors in ponds of 600 gallons or more will stay much cooler than comets in a 20 gallon tank.

Comets need much more room than a 30 gallon bin.

I would bring back the comets and look for the fancy goldfish if keeping water cool is an issue.

Thank All for your advices. My gold fishes are doing great. And also ever since since I added the filter they are all lively. They swim around the tank. Sometimes having fun taking a ride from the air coming from the air stone.

One thing I am now confuse about is feeding them. To me it seems like they are always hungry. I would feed them and they would eat. And whatever is left over (in 3-5 minutes)I take it out of the tank as instructed from the feeding guide. But in a few minutes you would see the gold fish at the bottom trying to eat something then to the top trying to eat something. It seems like they are always looking for food with the mouth open trying to suck something in. So should I feed them again or they will be ok? And so far I only feed them once a day.

Feeding Fish once a day is fine. Some ish will eat all the time like goldfish. They are poop machines.The less they eat the less they'll poop and the less water quality problems you'll have. As long as the schedule you use is not causing any aggression then don't worry about it. The high water temp. also speeds up the fishes metabolism, this encourages eating. This "can" also cause aggresion.