? diatom outbreak in QT

[squealy]

I've had my 20 gal QT up and running about 5 weeks with a wrasse in it. Last week the water became somewhat cloudy and brownish, and brown "algae" started accumulating on the glass. I assume these are diatoms. Question. Is this brownish, cloudy water bad for the fish? I did a 75% water change and the next day the water was the same. If it is bad, any suggestions how to clear up the water? I seem to remember something like this happening months ago in my other QT. Thanks.