Small particles in water

I have a tank, 70 liters, which is around 4 weeks old now. I believe
the tank is cycled now, all tests return good results. 3 fish in the
tank, a fantail, comet goldfish and a shubunkin. Yes I mixed the
wrong kinds of fish, but I never knew at the time, the fact they were
in the same tank at the shop never helped. But they do get on well,
feeding is not a problem. So maybe this time I was lucky.

Anyway, when the lights on the tank are off, the water looks clear,
however, when the lights on go you can see small particles floating
about, like bits of dust. The tank runs an Fluval 2 filter, I added
the optional polyester pad, that cleared the problem a little, but not
totally.

Would it be best to leave it for now, see what happens, or use one of
the chemical products which are meant to join these particles together
allowing them to be removed by the filter?

On the Fluval 2 filter I have, there is an "air flow control"
basically the top of the filter is out of the water, you can open this
to allow air in, this is then released into the tank for aeration - I
currently have this on so the water gets that additional aeration from
the bubbles, is that a good idea for my fish, or to leave it?

Jon.