This morning, Im looking at my fish Mako, and a huge portion of his tail fin is missing, with some frays left dangling. He had a previous tear there that had almost healed.
Yesterday, I was exercising him with a mirror. I didn't supervise him, but his tail was fine after.
The past few days, he's been stressed (Pale face, stress lines along the body), but no abnormal movement, eye cloudiness, or strange behaviour. He is not stressed while exercising, eating, or during the day. Only first thing in the morning and sometimes at night, for no particular reason. His colouring turns back to normal when he sees me. He exercised, ate, and interacted with me normally and didn't act strange once.
His being stressed only occurred in the morning, and I have been exploring the possibility that he's afraid of the dark (His old tank was located in the living room, where there was always light. His new tank, which he's been in for three months, is located in my room and out of direct sunlight. His overhead light stopped working recently, so I leave a lamp on for him during the day. That is when I noticed his being stressed more often). This incident also was discovered first thing in the morning.
There is no blackness around the edges of his fins, no receding finnage, no burnt gills, no obvious signs of any other sickness. I would say it looks like a bite, except that this would be one massive bite, and in the seven months I've had him, he has never displayed a tail-biting habit before. He is swimming normally.
I have no reason to believe that this is fin rot.
His decorations are silk plants and a plastic colosseum, which has been tested (Run through a nylon stocking), and I have deemed safe. I understand that it is possible that he ripped his tail on the decoration.
I have a low-flow filter and thermostat heater, both are new and were put in the tank two weeks ago to replace the old ones I had. There is no suction for the filter, however I understand that he may have somehow injured himself on them. They have no edges, and the heater is not hot to the touch. Both have been working perfectly fine since put in.
His tank temperature is 26C, water quality is fine, nitrates and nitrites at 0. The water is barely hard though. His tail fin is curling, and I've been told that is caused by hard water, but I have a water softener. It doesn't last long, and seems to evaporate the day after I put it in the water. I'm currently looking into another brand of water softener.
I've prepared a batch of water with aquarium salt and bettafix to put in my quarantine tank. Do you guys think I should use it? And also, does anyone know what this is and what caused it? It's really sudden. I'll try to have pictures up. Thank you!