Why is my betta fish acting weird after a tank water change?

The shock of full water and temperature change may be causing the clinical signs you are seeing now. With any luck, he will adjust and go back to normal. in the future with Wolverine or any other Betta, you should try to avoid doing 100% water changes unless it is absolutely necessary. Cycling the water weekly by about 25% is the best thing you can do to preserve the water your fish are used to.

I would recommend to change his water again and make sure you have a water conditioner that is not expired. I would also recommend to upgrade Smitty to a 3-5 gallon aquarium with a filtration system and a light. These tanks tend to create a healthier environment because the water is moving. You could add in live plants to provide a little extra oxygen. I hope Smitty starts feeling better!

Did you change any of the plants or other features in the tank? That could be confusing him. It could also just be due to stress. Did you take him out while you changed the water or did you leave him in? Taking him out is recommended but it can still be stressful for him due to all the moving. Did you do a partial water change or a full change? Since I am not sure of what your procedure was it is hard for me to comment. I suggest you refer to this article about how to properly change the water to make sure you are doing it correctly. If you still have more questions please feel free to post again. ( https://goldfisho.com/goldfish-tank-water-changes/ )

Cold water can affect activity in fish. Bettas are tropical fish and do best between 78 F and 80 F. Check the water chemistry also. You can use a home test kit, such as the API Master Kit, or submit a water sample for testing. The local Petco may do this for free or for a small fee. Aim for an absence (0 ppm) of ammonia and nitrites with nitrates at 20ppm or less. Perform a partial water change if necessary to stabilize chemistry and/or to improve water quality. Use only conditioned water that is temperature matched to prevent stress or shock. Diet can influence color, so it is possible this is partly to blame for the color change. There are diets that can be fed to enhance colors.

Drop the water temperature to 80 F. Bettas do best between 78 F and 80 F. The higher temp likely did not cause her death, but warm water holds less oxygen than cooler water. She may have been ill or old. She does appear bloated in the image. It may have been dropsy or other infection. Monitor water chemistry using a liquid test kit. They are more accurate than the strips. Tou can also submit a water sample for testing. The local Petco may do this for free or for a small fee. Aim for 0 ppm ammonia and nitrites with nitrates at 20 ppm or less. Try to keep the pH as close to 7 as possible. Bettas can tolerate acidic water, but changes should be gradual to allow time for acclimation.

From your description, I think swim bladder disease is possible. There is a really thorough article that I will link below. When you check water parameters, it is important to make sure you also check nitrites and nitrates. Ideally, ammonia and nitrites should be zero and nitrates should be equal to or less than less than 40. If nitrates are zero, it indicates that your tank is not quite nitrogen cycled. If it is higher than 40, then you need to do at least a 50% water change. https://www.thesprucepets.com/swim-bladder-disorder-in-aquarium-fish-1381230

Recheck water chemistry to confirm 0 ppm ammonia and nitrites with nitrates at 20 ppm or less. A pH of 7 is fine and they can adjust to more acidic water as long as the change is gradual. Temperature should range between 78 F and 80 F. Plain aquarium salt (NOT table salt or epsom salt) can be used as a general tonic to reduce stress, improve gill function, treat minor infection and aid in recovery. Dissolve 1 teaspoon of salt per gallon of water in a cup or two of tank water and gently pour it into the tank. Perform partial water changes every couple of days to maintain water quality. Treat only the volume of water being replaced. Treat with aquarium salt for no more than 14 days then reassess the condition. An antibiotic, such as kanamycin or erythromycin, can be found in the aquarium section of the local Petco if the salt treatment doesn't help. Follow label directions carefully and remove the carbon filter insert during treatment if indicated.