Sudden death in fishes can be multiple causes but in your Tigrinnus case I would say that the main ones I could think of are fatty liver, water quality (too high pH) or lack of O2 in water. I could be 100% wrong in all my possibilities and could be a bacterial infection or something but that would be unlikely with no external signs in them or the other fishes.
The lumps in their back seem to be a postmortem change since they were not present when they were alive.
If you have more tigrinnus left, transfer them to another tank with brand new water in the 7.0 range pH. Also add a lot of powerheads at the surface to create excess water movement and dissolved O2. When I had Tigrinnus I would feed them very heavily one day and rest them for about 4 or 5 days. A few times I saw them act sluggish and breathing a bit harder than normal and I moved them to one of my round ponds where they got excess O2 and water movement via the waterfalls and powerheads. After a few days in there, they were fine. My pH comes out of the tap at about 8 but in the tanks it gets acid due to my driftwood and the normal fish waste that always acidifies the water.
I hope this info, although very little, helps you in stopping the mysterious deaths. You can still add an antibiotic as prevention but make sure it is mild enough for the fishes. I am very sorry to hear about this loss because those fishes were extremely beautiful.
Regards,
Ivan