I have got about thirty goldfish in a plastic cold water storage tank about
4 metres long 1metre deep and about one metre high.

It's full of sludgy mud at the bottom. normally they don't disturb the mud
and the water stays clear. lately though they have been rushing and
thrashing about and the tank water now just looks black.

I'm wondering what is the easiest way to clear out the mud at the bottom of
the tank. my first instinct would be to take a small bucket and keep
spooning out the sludge. but i wondering if all the sludge got really
stirred up, and this would certainly do it; whether the fish would suffer
discomfort, and whether their gills and breathing can filter out all the mud
particles, without any trouble? any suggestions to a novice most welcome.