I am very sorry about your fish.
I just wanted to chime in. Ich does not ALWAYS show itself as the tell-tale white bumps on the fish. Those white bumps are actually the stretched fish's skin - stretched over the adult parasite that has burrowed under the fish's skin. You are seeing the parasite in a stage of life when you can not control it. You have to wait until it is no longer in the fish to kill it.
That said, ich can exist in enormous concentrations in a tank - and the fish may not display the telltale white spots. Juvenile ich parasites are free swimming. Other parasites can encyst and exist, untouchable, in the gravel base of the tank.
Ich is opportunisitic. It will attack any fish, but typically is mostly seen when something else stresses or injures a fish. Cold water, water parameters being off, an older fish, a fish dealing with other parasites or bacteria, a fish that is stressed from a new environment, etc. are all susceptible to ich attacking. The ich may lay in wait in a tank for a long time, and, when the conditions are right, they attack.
You rae not seeing any ich at this time on your other fish. But you can be sure that there is a substantial population of them in your tank. As ling as your other fish remains healthy and happy, the ich will simply remain in the background. But if anything changes or slips, they may attack and kill again.
A fish can actually carry quite a load of ich parasites and never show the tell tale white spots.....
I strongly encourage you to do a strenuous treatment for ich - cleaning the tank, warming the water, using treatments or salt to kill the parasite. Tub to tub will also leave the majority behind, while you clean the main tank......