Well it is hard to say exactly, but something must have spiked in your tank. If your tank is still cloudy, then you need to change the water again. It could be that your tank crashed and you are going to have to recycle it. Sometimes this can happen for a variety of reasons. Of course all mine are assumptions, since I don't know what kind of fish you have, or how many, what you feed them etc.
Your fish may have grown over the last 6 months, which means you are feeding more and they are polluting more, your filter isn't large enough, or you were not changing your water enough. A fish could have gotten behind the rocks and died, and this polluted your tank. The fish you have chosen may not go together, and aggression could have caused a death.
Right now it is going to be best to get your tank back on track before you add more fish. you can't change your water too much, and clean your filter again. And hopefully it will clear up for you.