Depends why the water is cloudy.
Is it just bits of fish food, poop, plant debris and silt from the sand? In this case buy some new cheap filter floss and put it in your filter. If it doesnt remove it, you need a bigger filter or supplement it with another small one for mechanical filtration.
If the water is cloudy due to a bacterial bloom or some kind of chemical colouring the water, add some carbon to the filter (but remove it after a day or two, as it gets used up fast). If it is a bacterial bloom, you'd be better of waiting it out and doing water changes, or IME adding a bottles worth of one of those off-the-shelf tank cycling products often clears it up within a day or two.
You could also use Seachem Purigen as an alternative to carbon, its much more effective but also far more expensive and a bit trickier to use. A waste of time for a freshwater tank IMO, it will remove more DOCs and trace elements because it works longer/better than carbon which isnt necessarily a good thing. Also , for the price of it you would be cheaper getting an internal filter and stuffing it with filter wool.
