Hi and welcome to the forum
That is cool and gross and really interesting. It looks like a black head pimple and I wanna pop it
How often does it happen?
Do you do anything before it happens (eg: a big water change, clean filter, stir the gravel up, anything)?
Does it appear in the same spot each time?
It's either a parasitic worm or a protozoan or something similar. I would treat all the fish with Praziquantel and then Levamisole. Don't use boht medications at the same time. If it's a worm those 2 medications should kill it. If they don't work and it pops up again after treatment, then move the fish into a quarantine tank and treat it with Metronidazole. If that doesn't work then the fish will have to live with it.
API General Cure contains Praziquantel and Metronidazole so would be the first choice if you can get it. Use it in a quarantine tank because the Metronidazole will wipe out the filter bacteria.
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To work out the volume of water in the tank:
measure length x width x height in cm.
divide by 1000.
= volume in litres.
When you measure the height, measure from the top of the substrate to the top of the water level.
There is a calculator/ converter in the "How To Tips" at the top of this page that will let you convert litres to gallons if you need it.
Remove carbon from the filter before treating or it will absorb the medication and stop it working.
Wipe the inside of the glass down with a clean fish sponge. Do a 75% water change and gravel clean the substrate. Clean the filter if it hasn't been done in the last 2 weeks. Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Increase surface turbulence/ aeration when using medications because they reduce the dissolved oxygen in the water.