I am not saying that UV was the answer, only that I used it.
A little more background of my experience.
I had just bought a new derasa and added it to the tank. I then experienced the die off of around 9 clams. I lost all except a smaller hippo. Unfortunately I hadn't found any threads on this when this was happening, so I had my friend hold the hippo to try and save it from whatever was going through my tank.
A short while later his squamosa died in the same tank the hippo was in. Seeing that the hippo never had any real issues during this entire time, I took it back. It has stayed healthy and has been growing well. I had run the UV on my tank for about a month before putting the hippo back in. And continued running the UV for another month or two.
After all that I tested with a maxima. After about 3 weeks of it doing well in the tank (better then it's previous tank actually). I then added another max and a derasa. All clams are doing well and showing good growth.
I still don't think this is the answer, just what I did. And knowing that the hippo was a carrier for whatever it was. And that I have it back in my tank, and doing well. Says that I have hopefully conquered this in my system.