The tank is 36x24x24, the sump is 24x12x15, about 350L of water in
total, I think.

There is indeed a UV unit in there, also a fairly recent addition, I
added it after reading "The reef aquarium" vol 3 by J.Sprung (Excellent
reference book, BTW). he says they have "many advantages and no
disadvantages" & I reakon he knows his stuff!

I think the desicion to stop water changes (I did originally do them)
came more from my growing understanding of the reef chemistry and the
nitrogen cycle rather than "I've got all the kit, I don't need water
changes" type of thinking.

Might be better to stand the question on it's head and ask those who do
water changes "what do you think they achieve". I don't need to remove
NO3 (It varies between 0 & 5ppm) or PO4 or anything else I can think of
right now. If adding trace elements is your reason, then you're
assuming that the salt mix you buy has the correct trace elements in it
(Some don't event get the Ca & alk. right).

BTW: It's not laziness either, I also have 4 FW tanks & they get 20%
changed every 2 weeks. Anyone got any live rock that works in FW?