Wow, you have a big tank! If I read the original post correctly, you have about a 400 gallon tank and 126 watts of UV. Also, your UV is plumbed from the sump. You referenced the big dino thread as well. After reading that thread, and others as well (and some personal experiences unfortunately), it seems that you increase your chances of wiping these suckers out if you plumbed your UV to and from your display tank. Your UV sizing seems to be on that cusp of 1 watt per every 3 gallons of water that seems to be the minimum effective size as well. Have you tried going to and from DT?
Thanks for the post. It does make sense to go from DT to DT with my UV. At one point I also had a UV running a loop in my sump. In the future if things start to go backwards again I would probably add a dedicated DT UV.
When doing some maintenance the other day I noticed one of my Ecotech MP40 wet sides was rusting. I think this has possibly been contributing to the dinos (I've seen a few posts where people believe this to be true. It also explains why blooms are often associated with rowaphos or other iron based medium). But I'm not certain.