how frequent do i change my water

"dj" wrote in message
ups.com...
hi guys,
just wondering how frequent i should be changing my tank water, it is
350 L, i've got live rock, corals and 8 fish. i have been changing
three quarters of it every fortnight but recently read that some people
change a third of their water every 3 months. i have fresh sea water
delivered to my house and i store it in a 1000L drum outside with a
powerhead for movement. my water quality is good and have been
sucessfully doing this for the past 5 years. i am just wondering if i
have been over doing it.

also, i have these strange worm-like things that build a cocoon like
tunnel on my live rock and they go onto my stoney corals and eat the
polyps. i usually just try and scrap them off the rock but it doesn't
seem to stop them from appearing. there aren't that many of them but
they are a pain in the arse. i never see the "thing" i only every see
the tunnel. it can actually tunnel through the rock.
hope you can help