one of my fish had whitespot it died now another of my fish is acting weird what...

...do i do? one of my fish had whitespot, and then just became seriously ill.. laying at the bottom of the tank and not moving to eat.
that fish died yesturday, so i cleaned out 75% of the water and put the fish back in.
since then, another of my fish has started twitching and doesn't look very happy atall?
what should i do and is there anything i can buy if the fish doesn't get better?