I have been battling periodic RTNing of my acros for quite some time now. It has not been isolated to a single tank, rather both of my tanks with them. I have had many problems (low light, stuck auto-feeder, power outages….etc), so other factors that impact health have been lost in the weeds. I am down to just my 210g with acros now. It is just finally to settle in with most starting to cover grow over the plug. The past probably 3-4 water changes I’ve noticed a couple frags RTNing each time, but was only able to trace it when I looked back on my notes. I don’t do big changes as I’m “a lazy”, between 10-20 gallons. It seems that something during the water change process is likely to blame. Investigation so far: I have been using an old Rio submersible to mix up my salt. Thinking that there maybe a seal was bad and getting some contact with copper or other heavy metals, I switched out the pump. I tested the water for copper. Nothing but even if there was some there, it would prob be below detectable range. Vinegar washing the Brute container. I use I/O so I do get the brown scum that tends to accumulate and which I neglect. Switched out the sediment/carbon/DI on my RO. I was still reading 3 ppm to the DI and 0 ppm out, but I do live in Minneapolis, so there is chloramine. I used one of the dip tests, but didn’t see chlorine or chloramine in the water change water. I ordered a stick TDS meter, but based on the DI resin not depleting very quickly, I wouldn’t think this is it. I don’t always check the parameters of my change water other than obviously SG, but I did test alk on this last batch. 10.1 dkh. Maybe a little higher than I would like, but it didn’t budge my tank alk. N/D on phosphate or nitrate. I suppose I could look at ammonia, but I don't know where it would be coming from. The heater is getting up to the proper temp, and there aren’t any visible signs of wear or damage. I let the salt agitate at least overnight and will turn it back on for 30 minutes before doing the w/c. Current tank parameters: Ca 440 Alk 8.6 dkh Mg 1300 Nitrate N/D per Red Sea kit (trying to dose some aminos to get that up) Phosphate N/D per Hanna low range phosphate pH 8.0-8.15 Nutrient export/filtration: RO Regal 300 int ATS Carbon/ GFO 1 cup of ROX carbon- ¾ cup GFO in dual reactor changed out every 3 or so weeks. Anything else you would investigate if this was happening to you?