For jumblepod
The link in your original post to the picture doesn't work. Can you post another picture of the sick fish?
Black patching is usually cell damage caused by chemical burns. It can be caused by ammonia, nitrite, or really high levels of nitrate, too much medication, or medication being used for too long. Chlorine or chloramine in the new water that is added to the tank is also a common cause.
The bloated bellies could just be fat fish. Generally if there is an internal bacterial or protozoan infection then the fish will stop eating and often the scales will stick out. Fish that develop internal infections get fat really rapidly, within 12-24hours.
Fish Tuberculosis (TB) doesn't make a fish fat until an organ fails/ ruptures. Then the fish bloats up over night and dies within a few days. They also do a stringy white poo and stop feeding.
Cancer is unlikely for several reasons. It progresses rapidly in small animals/fish and would have probably killed the fish within a month or so. For more than one fish to develop cancer in a tank the fish would have had to be poisoned by something at the same time. Then all the fish that were poisoned would develop the same symptoms within a few days of each other and start to die at the same time. You have 2 sick tetras and 1 healthy tetra (of the same species). This makes it unlikely to be cancer.
If your fish are feeding normally and swimming around without any problems, and doing normal poo, then I would say they have been exposed to some chemicals and probably still are being exposed to it if the black is getting worse. Are you adding any plant fertiliser? If so then make sure you don't overdose. I notice you used "Interpet internal anti-bacterial" and you slightly overdosed with that. This could be a part of the problem. Medications should only be used when necessary and never overdosed. They often have toxic chemicals in like Formalin/ Formaldehyde, Malachite Green and other substances. These are all extremely toxic to any living thing and the chemical companies have put dosages on the bottles to prevent the tank inhabitants from being poisoned by overdosing.
If the fish have been poisoned, as I suspect, then the black patching will take a few weeks, maybe longer, before it goes away. It is like a bruise on your arm. It takes time for the area to heal up and the bruising to slowly go.
In the mean time I would suggest you stop treating the fish with any medication or plant fertiliser, and do some 30% water changes and gravel cleans each day for a week. This will help to dilute any chemical residue in the tank.
Make sure any new tap water is free of chlorine/ chloramine, and has a similar temperature and PH to the tank before it goes into the tank. Perhaps even double or triple dose with dechlorinator to make sure the new water is free of chlorine. Make the water up in a clean bucket before adding it to the tank. Add your dechlorinator and aerate the water for 10-30minutes (longer if possible) before using it.
Make sure any equipment used on the tank, (buckets, hoses, etc) are free of chemicals and haven’t been used in the laundry or for washing the floor or car. If you don’t already have buckets that are used specifically for the fish tank, then I would suggest getting one and writing “Fish Only” on the bucket. Then keep all your fish cleaning stuff in that bucket and make sure no-one uses it for anything else.
Make sure no-one is painting in the room, using aerosol sprays like perfumes, deodorants, etc, and your hands/ arms are free of grease, cream or chemicals before you feed the fish or do work on the tank.
To work out the volume of water in the tank
measure Length x Width x Height in cm
divide by 1000
equals volume in litres
When measuring the height, measure from the top of the gravel to the top of the water level. If you have big rocks or driftwood in the tank, remove them before measuring the height.
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