bacterial problem due to live artemia feeding?



I have been breeding clowns and raising fry with reasonable success for a little while now and have noticed that since I started using r/o di water as replacement water the perc fry are not doing so well. I raise the fry with broodstock water and start using 'new' replacement water around day seven for the percs and day ten for the yellow striped maroons. I co culture as well now which seems to improve survival rates. I keep the tanks at 80 degrees. Salinity slowly adjusted from 23 to 18 over a period of one day the day of hatching. I use tap water with ph of 8. The new saltwater is also 8. Up until just recently this method seemed to work well. No perc babies survive past two days after I start using new water. I clean and disinfect equipment and tanks as usual so I can't see that being a problem. I have tested the broodstock tank and found no issues after testing for ammonnia, nitrite, nitrate, ph, kh,. I am wondering if it is an bacterial issue because right around the time the problem started I had begun feeding live baby brine shrimp to the mandarines that are in a tank connected to the system. I understand that they can carry bacteria which could cause problems. I have also noticed that this problem is limited to the perc fry-the maroons do fine. Also, the water in the fry tank with perc eggs becomes quite cloudy shortly after the babies hatch and this has never happened with the maroons.(another indicator of bacterial issues maybe) I am reinstalling a u.v. sterilizer. Wondering if anyone has had any similar experiences.