[duncaholic]

IME starting my larvae on oto is usually around day 4-5 and this seems to work well. Although I still keep a small rot population through day 8-9. By then they have usually made the switch.
IDK if you should remove the blacking on the sides earlier than meta, although most of the breeders on this site seem to have very good succes doing so. My last hatch has been in a black tub, and has done very well. Compared with the 5-6 before that that experienced 100% mortality, I think eliminating side lighting has worked well for me. There have been studies that strongly suggest clown larvae remain highly sensitive to light until they are past meta. But at 18 dyas, they should be there.
Water changes in larval systems can be tricky to say the least. Even if you match the salinity/yemperature parms well, newly mixed synthetic salt water IME can be an absolute killer. IDK why, but everytime I used to use new water in ant tank, it would stress it heavily. Now I pre-mix water at least 24 hours in advance and keep a strong current moving in it the entire time, but still it can be dangerous. Especially if the PH is much different. PH seems to me to affect the impact of other chemistry issues.
Even if you are using parent tank water, a 90% change is quite drastic. I hope that you added the new water very slowly. Most people say no more than one drop per second, and I think that's a good rule of thumb.
Hope that your babies are doing well. post some pics if you can!!!