Hi,

I've just had another, more detailed, look under the microscope at the
organism which seems to be blooming in my tank and seems very similar
to David's and made some more detailed drawings. Then I sent the
drawings to a microbiologist who has identified them as (probably) a
stalked peritrich, which is a kind of ciliate protozoan. I think it
should be harmless in the aquarium and should not cause any disease in
fish so I'm breathing a big sigh of relief. Of course, I don't know
if this is the same organism as David's or not, but, certainly on the
surface, it seem to be good news!

Hope this helps,

Muddy