Jonny
If they are zebra's, its got to be bad news for the country if more so your area. I suppose it's inevitable that they will spreed. If the water isn't like tap water, it soon will be as they will filter it clear of any colour.
If they are zebra's, there should be clumps of them latched onto rocks, stones, swan mussels, each other and they are dark with a dull stripey look in my experience.
I suspect some waters will suit them far better. Is the water a stoney bottom?