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I have a 48in x 15in x 12in (29.6 imp gallons) with approx 1/2in of swimming pool filter sand, 2 medium sized pieces of bogwood and 4 aquatic plants that have been in the tank a 3-4 months (no idea what they are) Tank has been up and running for about 12 months.
Filtration is by 1 x 380 lph and 1 x 500 lph sponge filters.
Water is HMA'ed and stored for several days before use, approx 25% is changed weekly.
Ammonia 0 ppm
Nitrite 0 ppm
Nitate 40 ppm (the same as local tapwater)
Temp 25.9C (+/- 1C)
Occupants are/were. . .
5 x 3-4 inch Krobia sp 'Xingu red cheek'
1 x 4in Hemiancistrus guahiborum L106
1 x 3in Leporacanthicus triactis L091
1 x 5in Peckoltia sabaji L075
4 x 2in Peckoltia vittata L015
3 x 1.5in silver dollars

No fish or anything else has been introduced to the tank for about 4-5 weeks (last thing was the silver dollars)
Last week 2 of the Krobia had slightly cloudly eye (one eye each) and I started treating with API Melafix.
Instructions are 5ml per 38 litres daily for 7 days and a 25% water change at the end of the 7 days.
I dosed for 25 gallons for displacement of sand and bogwood.
I worked this out at 15ml daily. The 15ml was measured by a new syringe bought from a local chemist.
Everything seemed fine, today was the 7th day.
Today I found ALL of the L-numbers dead, the krobia hiding with clamped fins and the silver dollars gasping at the surface.
I did an immediate 75% water change with stored HMA water (same temp as tank water)and the Krobia seemed to perk up slightly but still have cloudy eyes, the silver dollars really improved.

Nothing had changed in the tank for months except for the introduction of the Melafix.

Is there a known problem with Melafix ?

This one has really got me scratching my head in confusion

Sorry for your losses...

I've heard that melafix affects the oxygen level/uptake in the water.

I would also point out that 25% water changes on that size tank and stock sounds pretty low - and I'm surprised with that stock that your nitrate level isn't rising quite a bit above the 40 ppm of your tap-water. I counted about 35 inches of fish, and the guideline is to have no more than 1 inch per gallon.

I'm also a bit surprised that all the plecos died at once, but perhaps one of them passed and started a chain reaction of ammonia rising in the tank. I presume your ammonia measure isn't from just when you found the dead fish in the tank?

--
Mats

OMG. That was a nasty shock.

I used Melafix recently for the same thing. It had some effect, but didn't clear the cloudy eye. After a big water change I swopped to using Myxazin, it worked. All the time I had the power filter outlet above the surface to create turbulence.

I use both Melafix (5% solution) and Pimafix (2.5% solution)n with real nice results of speedy healing. Yet, I've never tried to stick to the dosing they suggest: 5 mL to 10 US gal (38 liters indeed). I start small, like 10x under and add approximately the same amount every day for 3-7 days and then a 50% water change. Never noticed any significant changes in the fish behavior or habits. Never used on small fishes though but mostly in my 2500 (indoor) and 12,000 (outdoor) US gal ponds.

I have tried Melafix and Pimafix and find they are not nearly as effective as drugs designed to disrupt cellular functions of specific disease organisms. I would not recommend them for any case of serious disease.
I belong to the camp which feels I don't care if there are chemicals in it as long as my lettuce is crisp.
Melafix may be a good adjunct to a black water tonic if you are trying to simulate the complex organic compounds present in black water.