Give it plenty of time in acclimation. I drip acclimate my fish for 3 hours. Rob @ SWSW said it only takes an hour, or else you have to use an airstone. I just think the longer they have to get accustomed to our water the better. I have lost very few new fish since I started using RO water and a longer acclimation period. Also, quarantine the new guy. That way you know ich isn't going to invade your DT and it gives the BH a place to relax and destress from shipping. My new BH also spent the first 2 or 3 days on his side, under a rock. I just kept testing the water and left him alone. After 3 days it started eating and coming out more often. Now it just begs for food when I get near the tank, instead of hiding.