For what it's worth, Lymphocystis is viral and usually found on fish living in poor conditions. This could mean that the Live Rock you have in the tank may be leeching something as well as possibly the tank decor being too sparse. Lack of security causes the fish to get stressed which is the first cause of diseases. The fact that the fish last for the first 2 weeks okay before deteriorating sounds like the environment they are being housed in is not sound enough for long term use. Since you mentioned a number of different types of fish all having issues, I would have to think it's not the acclimating ( I rarely saw fish poorly acclimated and surviving 2-4 weeks) but the set up. So let's dig deeper.
I see you have what appears to be good water flow but what about oxygenation? Have you tested for dissolved oxygen?
Recalling a situation at a customer's house, do you have anyone cleaning the house or area near the tank with any kind of chemicals that may be getting into the water? ( My customer's housekeeper was bleaching the floor right in front of his tank and the airpump just sucked it in and routinely killed his fish every 6 weeks like clockwork until I visited his house and asked about it.
)
As I was getting out of the marine fish importing biz back in 2000, it was becoming a common occurrence for fish lice to be on many of the fish coming in from the Pacific. A simple freshwater bath after acclimation and just prior release into the tank was solving a lot of those issues. Have you tried FW dipping your fish?
Let's start with these scenarios.