I have no idea what's causing these fishes to die. It doesn't look like velvet or ich. I thought it was Brooklynella but it doesn't really look like a either because the fish are all dying by days apart and not all the fish are affected I'll post some pictures.
Using an inexpensive jeweler's loupe or microscope to help in the ID would have been useful...but there are only one or two folks on here I can remember going to that level for their fish.
In the future, you might try running a search on the forum back over
@Paul B's posts or shooting him a PM. Can't recall off-hand who else.
Also, these things usually come up for fish because of a compromised immune system. Hippo's are always the first to display signs of stress (and of course then infection), but the stress has to be beyond a certain level for the whole tank to tip toward infection.
Before restoring any (or many) fish to the display, you should take measures to resolve whatever stressors were around to degrade the situation in the first place. This could be everything from belligerent tankmakes, to crowded conditions, to poor system temperature control, to poor nutrition – they all add up.
Consider giving this a read:
Stress - Its Role in Fish Disease
That last one (nutrition) is usually a biggie....without excellent nutrition, a fish with a
thin slime coat like the Hippo tank stands poor odds of ever getting healthy.
Live foods and whole-frozen sea foods are really best, but what do you have available and what have you been feeding so far? Dry foods should be supplemented/soaked with a high quality fish oil just like you take.