So sorry to read this
Apologies in advance for the following - I'd've sent via pm but I cant seem to find that feature - I can edit it out if you'd prefer.
It could be "columnaris" - there is a form that overwhelms the gills before you see any outward signs, fish die rapidly as you witnessed.
There's not much you can do but offer supportive care, ie excellent water quality, maximized oxygen levels, very dim lighting, minimize external stressors (eg, vibration, fast/quick movements near the tank), feed sparingly with foods such as daphia & brine shrimp (rather than bloodworms or other "richer" foods) ...
after a couple weeks with no more new deaths, you might feed an antibiotic-laced food (as always this is controversial, make sure you choose an antibiotic that has actually been shown to have efficacy in living fish in aquaria, eg kanamycin, neomycin sulfate, sulfathiazole are all relatively palatable & broad spectrum enough to be supportive: note this is not about treating residual columnaris but suppressing other secondary bacterial counts) for 3-5 days, then 2 weeks off, then antiparasitic-laced food (levamisol gets my vote, metronidazole is a poor second, again this is about reducing numbers of opportunistic parasites which have increased while fish were stressed) for 5-7 days.
Again choosing to treat/not-treat is 6 of one vs half a dozen of the other ... I generally avoid antibiotics as they also upset "beneficial" bacterial populations, levamisol is a very effective antiparasitic while also considered to have immune "boosting" effects (as a bath it's stressful - as are most medicated baths).
But there are some external parasites that still respond better to formalin than any other medication (but it needs to be fresh & stored properly & dosed appropriately & care is always needed to maximize oxygen levels & daily water changes. Note that every bath medication affects the filter bacteria to some degree).
I keep a second planted tank that I run as a quarantine type tank, it's no where near as "clean" as a bare tank, but fish are much less stressed & in fish, stress is strongly immunosuppressive.
A quarantine time of minimum 2 weeks & preferably 4 weeks is recommended.
It's also recommended to add some of your main tank fish into the Q-tank for 2 weeks before finally introducing all the "new" fish into your main tank.
Most of the time, impulsively adding fish works out just fine.