Dying Black Neon After Using Melafix

My bristlenose pleco was getting a torn/ripped tail again (despite the fact that it had slowly been growing back recently) so I decided to do another round of Melafix in my aquarium. I have used Melafix in that aquarium many times before without any issues (4 times in the last year in fact) for 1-2 weeks a time. I used the same bottle (it doesn't expire until 2023) and the same dose that I usually do. If anything, the dose would have probably worked out a lower dose than normal as I no longer use an internal filter so more water volume, there is some carbon in the filter which I wasn't able to remove and the water level was quite a bit higher than normal this week.

The only thing that has changed since I last used Melafix is the filtration. I changed the internal filter for an external one 2 months ago. I did this carefully and it did not uncycle the tank at all. All water parameters are fine, pH hasn't changed.

I put the Melafix in this morning and checked on my fish a few times and everything seemed fine (I always get nervous about using treatments in the tank). Went to check on them at lunchtime and I couldn't find a black neon tetra. I then realised it was near the surface of the water, gasping for air. It then completely lost its buoyancy, and I can't find it now. Worried it might be dead or dying. Could the Melafix have done this? All other tetras and the bristlenose seem fine.

The black neon died unfortunately 

I had used Melafix with that same black neon several times before with no issues, so maybe it had recently developed some illness I was unaware of.

I would be a little nervous using salt with my bristlenose, and there are assassin snails in that tank which I am guessing I couldn't use salt with.

That's good to know, I always assumed salt was bad for snails. I actually have some API Aquarium Salt somewhere so I might give it a try. I do recall using it in the past (before I got my bristlenose) and actually removing the assassin snails from my tank first as I thought it would harm them! I certainly won't be using Melafix again!