Why does my tropical tetra have a cut fin? Is it fin rot or injury?

This presentation could be compatible with fin rot, which is a bacterial or fungal infection. It is unlikely that the primary cause is traumatic, unless Tank is itchy (that could be compatible with a bacterial infection anyway) and he is scratching himself constantly. The approach must be logic anyway starting from water quality check and treating with some antibiotics. Begin by submitting a water sample for quality testing. Most pet stores will do this for free or a nominal fee. If water quality is good and there are no major imbalances, water should be treated with antibiotics. Please do not hesitate to contact us again on the forum or by requesting a consultation if you have any more questions or to discuss it any further.

Thanks for reaching out about your fish. Water quality or aggression by other fish should be considered. Testing water quality for pH, ammonia, nitrite and nitrate, correcting any problems. The fins should re-grow is the problem is corrected. Good luck.

Thank you for reaching out about your molly. Unfortunately, the fins are not very good at regenerating themselves completely, so your fish may always have a bit of a scar/missing parts of the fin. I also see that there is a potential scar/white area on the side of the body? These two together may be from this fish getting caught on something within the tank, or from trying to wedge himself into small areas where he can get caught. It would be less likely to be an infectious agent, such as a bacteria or parasite, or an environmental/water issue as the other fish appear to look okay in the photos from what I can tell. I would suggest trying to look very closely at the tank furniture and the fish's behavior to try to identify where it may be getting injured. Alternatively, you can partition out a portion of the tank for this fish to reside in for a period of time to allow the injuries to heal. This would also allow you to be able to rule out whether it is trauma from one of the other fish as well. I hope this gives you a place to start, and thanks for using Petco Pet Education Center, formerly Petcoach.

Keep her separated and maintain pristine water quality to aid in recovery. You can add some StressCoat and 1 teaspoon of plain aquarium salt (not table or Epsom salt) per gallon of water. Treat with salt for 14 days maximum then reassess fin condition. Once she has healed, return her to the tank. Try to keep them in groups of 4 or more.

Thanks for reaching out about Lucky. First, check water quality for pH, ammonia, nitrites and nitrates. Correct any problems. Is there any other fish in the tank that could be aggressive? Bacterial infections can attack the fins/skin, and if the lesions progress, a veterinary visit is recommended to assess Lucky's health and work on a diagnostic/treatment plan. Any new fish might have introduced parasites. Good luck.

Thanks for reaching out about Ember and including photos. Not sure if any lesion are recognized. If concerns continue, a veterinary visit is recommended for a possible skin scrape and microscopic exam. Maintain pristine water quality and provide a good diet. Good luck.

Thanks for reaching out about Kai and including photos. It is difficult to determine is this is a full-thickness hole by the pictures. Tails tend to regenerate so monitor to make sure it doesn't get worse. Test and correct any water quality concerns. If other spots occur, testing for parasites by a veterinarian is recommended. Good luck.