Yet another concerned goldfish owner seeking help.......

Morning all,

I have a 'Biorb' set up here made by Reef One. http://www.reef-one.co.uk/

Mine looks just the same as the one on the home page and I have four fish in
it.

To the point, one of them keeps hanging with its head up and tail right down
until it then gives a few swishes of its tail to right itself. The others in
comparison seem ok but are just hanging around looking lost. I could always
take a water sample to my local shop who will test it for free, but any
pointers in the mean time please?
The one that 'hangs' (4 yr. old daughter calls her Erica!) seems a bit
rotund around the body and I wonder if it is constipated.
I only feed once a day, using Hikari Staple floating pellets. (32% protein,
4.8% oil, 3.2% fibre, 10 Ash). Is this food suitable please?
The water is very clear looking. Also, in the slighter dimmer weather we are
now having in the UK, should I be leaving the light on for a certain period?

Questions, question. Hope someone here can hold my hand for a bit 'till I
get this sorted.

Thanks and best wishes, Nick, U.K.