Is there any way you can put one of them into another tank or maybe upgrade to a 5 gallon and divide it? It's perfect for one betta but not so good for two.
My nitrates and trites were really low in the safe zone. My PH was pretty low so i used a PH increase solution. and everything else was in a good place.
did you use a dip sick or liquid test kit with the test tubes? ammonia and nitrIte should be at 0 or as close as you can get it, and i think nitrAtes are supposed to be under 40
The halfmoon may of been snacking on his fins - which can lead to fin rot.
Did it happen in a few hours/overnight or have they gradually been getting shorter?
Tail rot or fin rot
•Symptoms: Betta’s fins and/or tail seem to be getting shorter and shorter or they seem to be falling apart and dissolving, Black or red along the edge of the betta’s fins/tail, Bloody tips, Behavior may not change
Treatment: Conservative: Treat with Aq.Salt at 1 tsp/gal. Increase water changes to 100% daily. Replace accurate amount of salt following water changes. Add Stress Coat to help repair tissue. If there is little to no improvement within the first 5 days, you can increase the salt dosage gradually to 2tsp/gal but do not continue any salt treatments past 10 days. Medication: If Conservative treatment is ineffective use API Tetracycline, API Fungus Cure, API Triple Sulfa, OR API Erythromycin. Also add Stress Coat to help regrowth. Continue until fins/tail stop receding and start showing some new growth.
•Symptoms: Not to be confused with Fin/Tail Rot, Fin or Tail biting is when a betta bites his own tail, tearing off pieces. Fins will be ragged, they will look like there are chunks missing, Fin loss can occur overnight, The tips of the fins are clean, no black or red edging, You may spot him swimming in circles, chasing his tail, There may be no other behavioral change
Bettas bite their own tails out of boredom or misplaced aggression. To cure his boredom, try getting him a larger tank, move things around in the tank to spark his interest, put a mirror up to the tank a few times per day to let him exercise, offer a variety of foods, get him some tank mates (research into this first though), or put his tank next to another betta’s tank ect… Owners of fin biters often have to deal with their tendency for a lifetime but do what you can to draw his attention from his fins. When your betta does have a fin biting episode, it is important to keep his water extra clean to prevent infection. You can treat him for a few days with Aquarium Salt until you see regrowth but you do not want to overuse it. Using Stress Coat as the water conditioner will help fin regrowth.
I had a red CT that looks lookd like that :( He was in a 5 gallon, got water changes 2X a week, stress coat, heater...nothing worked..even tried tetracyclin and maracyn thinking maybe its something bacterial. I was then thinking ammonia burns but I dont see how with 2 water changes a week and one being 100% (non filtered tank) or fin bitting....