How many fish did you add? You basically added fish to a tank that has no filtration. the undergravel filter needs time for it to start really working.
My guess is that youve added fish to the new tank but the filters cannot keep up with the waste that the fish are producing..causing cloudy water..this is known as cycling
Cycling a tank is basically getting the filters working correctly...when you set up a new tank even if you have the best filters the water is still not really ready for fish. you need some type of biological filtration going on..there area few ways to do this...
1. the way you are. Adding fish and a undergravel filter..is a biological filter. the water flows thru the gravel and back up the tubes..left over food, fish poop
, etc..get trapped in the gravel causing bacteria to grow..this same bacteria actually cleans the water..to much gunk in the gravel can kill the bacteria. this is why people use the syphon thing to clean the gravel..usually once a month is good.
this is a good method, but can cause the death of some fish..since the water gets very dirty during this cycle..not as much visually dirty, but chemical wise..ammonia, P.H., nitrates.
2. Pet stores sell liquid and powder bacteria and enzimes that help speed up the filters
3. if you have a friend that has a established tank ask him if you can have a handfull of gravel from his tank..treat is as you would a fish..keeping it in the same water untill putting it in your tank..if you take it out and let it dry or rinse it under tap water you will kill any bacteria living on the gravel..this will help speed up the process.
What I see is that you have a brand new 20gal tank. with just an undergravel filter and a power head moving the water thru it correct? in my opinion you need more filtration. I would get one of those filters that hang on the back of the tank. they work great and will make having a fish tank so much easier. this filter combined with the undergravel filter should keep the water clean as can be. as long as you change the carbon, clean the bottom of the tank and do other maintenence.
Also...What type of gravel did you use...sometimes you need to rince the gravel off before you use it. that could be something else to think of..if its a natural stone gravel then you might have some cludyness from that..it shouldnt last long and go aways eventually.
And one more thing...you NEVER want to clean a whole tank out once its running. your killing everything thats working for you...meaning bacteria.
Are you using Stress Coat when adding new water to the tank? tap water has chlorine in it which is bad for the fish.
I have so many tips and info about this subject if you need some pointers let me know. I could probably go on and on about this but ill spare most of ya..heh
I just let more of my geekness out again...