Mucky looking tank

Hi there,

i was wondering if you could help me

i'm a new to the fish world and i just got my 1st tank. it's an 18 inch long
one that hold about 20 litres of water. with 7 freshwater fish in it i have
a underwater filter (mini filter), air stone, and a little tresure chest
running off an air pump too. i also have a light fixture thats flurecent
with a 15W bulb in it, i also took out 1/3 of the water and changed it
adding back the Ammonia Chlorine illuminator and "sure start"

but to me the tank still looks mucky, it has a light brown tinge to it. and
i notice alot of little whit pecks, or dots floating around the tank to. i
thought it would be nice and clear. my girlfriend has a tank to with smaller
fish, but smaller tank. holds about 14 litres. and her water is crystal
clear!

what can i do to clean up this muck? i was thinking of getting an unground
filter instead, that sits under the rocks? or one of the magnetic things you
put on the glass to help clean the inside.

whats do you think is causeing it? and why?

thanks for your time