Desperate - murky water

Wonder if anyone can help - I'm getting desperate - and the fish are
getting very hard to see :-(
My fluviall 3 filter (big tank) broke over 3 months ago. Replaced it
with another fluviall 3 but they've changed the design. The tank got
cloudy - new tank syndrome - which after 6 weeks went very green.
Realised I had an algae bloom - thats now cleared, but the tank is
just getting cloudier and cloudier. I've tested nitrites, amnonia and
pH and all seem OK. One fish got fin rot which I've treated but I am
concerned about the quality of the water. I've phoned the filter
manufacturers and the shop a few times and have been given consistent
advice, but its not working. I'm doing 20% water changes every two
days and using cycle (for new tank set ups) but its not working. I've
put in a fluviall 1 at the other end of the tank a few weeks ago but
its making no difference.
When I look inside the fluviall 3 it seems clean apart from at the top
of the sponge. Is it possible I've put it together incorrrectly? (Air
is coming out of it).
Is there anything else I can try? Anything I could have been doing
wrong?
Many many thanks in advance for any help.
Kate