I just got a new beta fish shipped to my house about 1 week ago. I acclimated him and did everything as noted by the

is he in a bowl or filtered  tank?

what is the water temp?

please give me 15 minutes to type answer

I am sorry Star is not himself.

Due to inbreeding Bettas have very weak immune   systems leading to fin and  tail rot.

Lethargy can be a sign of a few things and they are:

1) unfavorable water conditions

2) old age which for a betta would be anything 2+ years

3) disease

What I recommend be done is change 30% of the water in the tank using dechlorinated water that is the same temperature as the water already in the tank.

Verify that the water temperature is at the proper level for bettas which is 80f/26c degrees if not then slowly adjust it until it reaches the proper temperature.

Ammonia and nitrites must be zero

Nitrates 20 ppm or less.

PH 7

Aquarium salt should be added at the rate of one teaspoon per gallon of water in the tank. Salt can make a prolapse go back in, but it also helps with fin and tail rot.

The rot is usually caused  bacteria, so we'll also need medication to eliminate them.

Kanaplex, furan 2 or Maracyn oxy

here are a couple of other things you can do to support the regrowth of healthy fin tissue. These are safe with the medications. One is a supplement called VitaChem, which you'd probably have to order online. Here is a reputable source.

http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=4998

StressCoat (available in most pet stores) may also help. VitaChem and StressCoat can be used in conjunction with medicated food or medications in the water.

https://www.chewy.com/api-stress-coat-aquarium-water/dp/122094?utm_id=272255476&msclkid=e581ef57e7361e3369a4b1a9f38b994c&utm_source=bing&utm_medium=cpc&utm_campaign=Shopping-Product%20Targets-Consumables-NB&utm_term=4585307088206118&utm_content=Fish%20Water%20Conditioners

https://www.youtube.com/watch?v=1uGsQMNc5vw

I hope this information has been helpful. I will be standing by if you have other questions. Let me know if I can help further. Also, before signing off today, please take the time to use the star rating system at the top of the page to leave a rating for me (bottom of the page if using the app). A 5-star rating is appreciated as top notch answers are my focus. If you feel you have received anything less, please reply so that I may help until you’ve received the assistance you need. Until you issue a rating, the website will not compensate me for helping you. You will still be able to chat with me even after issuing a rating at no further charge on this thread.

Kind regards,

Terri