Please help: How to keep clean water in tank and other pro..

Hi!
I'm new in this hobby and need help... I have 21 lit. tank and 6 goldfish.
First, I cleaned tank, than put plants and sand in...
After that, i put water and fish... After 10 days, water is not clean as
before, and is getting worse...
3 Days before I get 2 fish from friend and one of them wont eat anything.
She has white edges of gills and looks like something is wrong with her...
She just running out of other fish... I tryed with friends food, but with no
results for her... others are ok...
So,
- What to do with food for that fish?
- Is she sick?
- Why water is not clear?
- How to dechloriate water?
- Why water in tank smells bad?
- How to measure quality of water (pH and stuff)
- Why I'm not buy a cat? :)))

Please, give me some advice...

P.S. Filter is with sponge and total no. of goldfish is 6 :))))

Best regards,

/DIK

I would recommend an even larger tank then Billy suggested. The general
rule of thumb for goldfish is 10 gal per fish (38 ltrs) So, with just
the goldfish, you'll need 230 ltrs.

Dechlorinating water is HUGELY important. Chlorine and chloramines can
kill some fish within hours. (or so I hear) You'll need to immediately
go to your local fish store and buy some conditioners. Ask for
something that neutralizes chlorine, chloramines and heavy metals. In
the U.S. I recommend Kordon's AmQuel and NovAqua together or Prime. I
don't know if they sell those where you are, but they are both quite
good.

Why you are there you'll need to pick up some test kits. These are
standard kits every fish owner needs and should not be considered an
optional investment. [Lots of new aquariests like to convince
themselves they don't need to test their water ;-) ]

You'll need:
pH
ammonia (salacylate test kit, comes with two reagent bottles instead of
one)
nitrite
nitrate
kh (carbonate hardness AKA buffering capacity)
gh (general hardness)

These are the basics. I don't keep goldfish, so if anyone knows of
other test kits necessary for goldies, do tell.

Oh, the cat question... I say cats are much easier then fish.
Welcome to the hobby... er should I say lifestyle? :-D