What size is your tank? 2.5 gallons
What temperature is your tank?
Does your tank have a filter? yes
Does your tank have an air stone or other type of aeration? no
Is your tank heated? no
What tank mates does your betta fish live with? He lives with another male betta (La, a halfmoon) they have a tank separator
What type of food do you feed your betta fish? Aqueon Betta Food pellets
How often do you feed your betta fish? daily
How often do you perform a water change? once a month
What percentage of the water do you change when you perform a water change? 100%
What type of additives do you add to the water when you perform a water change? Top fin betta water conditioner
Have you tested your water? If so, what are the following parameters?
Symptoms and Treatment
How has your betta fish's appearance changed? suddenly skinny
How has your betta fish's behavior changed? no change
When did you start noticing the symptoms? 5/28
Have you started treating your fish? If so, how? removed from shared tank for observation
Does your fish have any history of being ill? no
How old is your fish (approximately)?bought from pets mart 1 month ago
Try changing the tank water more often than once a month. In a 2.5 gal, it's ideal to do 1 50% and 1 100% weekly for just one betta. Try doing 2 75% changes weekly.
Also, spread feeding out into 2 different meals along with varied food. It helps avoid bloating and constipation. Depending on the activity level of your betta, you might need to feed a bit more or make sure that he is actually eating his food.
If he's constantly flaring, try to break his line of sight from your other male betta.
I would recommend upgrading to a 5 gallon tank and dividing that for both of your boys to give them ample room, decrease water changes that you have to do, and to give you more liberty for decorating. Another 2.5 gallon would work as well since 2.5 gallons is ideal for 1 betta.
I noticed your tank isn't heated.. fluctuating temps from night to day / house heating on/off, etc, might not be helping. There's some really affordable heaters available, it might be worth looking into one. It'll mean less sickness for your fish.
He may have internal parasites - what does his poo look like?
Well, it can depend on what you feed them, the fish's general health.. My healthy fish poo in little pellet-coloured blobs, occasionally a longer one but mostly blobs.
The only betta I've lost so far had internal parasites. His poo was always stringy and pale, or a proper colour but coiled up tight like a flat snail shell.
If your fish is in a separate tank, try removing all the substrate and watching what happens when he poos.
Being in cold water or water that changes temperature up and down won't be helping. Bettas hate change, and will often get ill even if that change is for the better in the long run.. constant temp changes can make them very stressed.
Once a month water changes in a 2.5 gallon tank is really not enough to give one fish good water quality needed to be healthy, let alone two of them. They'll be swimming in their own pee after a week, which is ammonia, which is poison to fish and -will- cause diseases as the ammonia eats away at their skin. Twice a week, one 50%, one 100%. That would be minimum for both the shared tank and the one Twi is in.
Try getting a heater, and changing his water twice a week. Look at his poo, see if it's brown and blobby (healthy looking) or stringy and pale/white which may indicate parasites.