Hi Thincat,
Thanks for the reply. My water parameters are stable and always have been (I use RO water @ <002ppm TDS), 1.025 SG, 0 ammonia/nitrate, and nitrates are approx 30ppm (no corals), pH 8.2. Temp is consistent. I feed high quality pellet/flake food usually, with the odd frozen as a treat. I will check for PO3 as well, as I have a bit of algae (not a huge amount).
Regarding the problems I've had with fish dying at each of my last 3 waterchanges, I'm going to recalibrate my TDS meter, make up some new RO, and I also have a fresh batch of salt to try, so I can at least rule out the new water. Is there anything else I should be testing for in the new water other than pH/temp/SG/TDS (before salt), to see if anything else has leeched into the water/bucket etc.?
If the new sw is okay, another issue may then be that I'm stirring up/upsetting something in my tank when I do a waterchange, although as discussed this isn't showing up as ammonia or anything that I've currently tested for, and things settle down within an hour or two. Again, is there anything I should be testing for here to see what's happening, that I'm not already?