have you tested the water?
is he in a bowl or filtered tank?
can you post a pic?
Cloudy water can indicate ammonia.
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
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 20ppm or less
Begin to treat the fish with a medicine called Betta Fix which can be found at most major pet stores. Follow the directions on the product label for usage/dosage instructionWhen a Betta first comes home he can be very stressed.
A new fish tank setup is always going to bring stress to the fish because its new. What is important to know is that stress puts a fish’s body at harm. Fish rarely get sick unless under stress levels. So if you can cut back on as much stress as possible, the better.
If you think your betta might not be eating due to stress then maybe try bringing some darkness to the tank to reduce stress. Cover the tank with a small blanket so the fish doesn’t have to witness all the movement outside of the tank. Even though he is inside the tank, he still gets stress from movement in his surrounding view. Darkness helps this stress tell he is more used to the new environment.Your Betta needs to get used to different food than he's used to. Try different brands till you find something he likes. Try soaking the pellets and blood worms first. Dry food/blood worms will swell in his belly and could cause swim bladder disfunction.
Try a different brand of pellets like New Life Spectrum for Betta's. Frozen blood worms and brine shrimp are better than dried, but don't use everyday.
However, if after a week or so your fish still isn’t eating, consider trying a different food just in case (but don’t freak out too much, as it takes several weeks for a Betta fish to starve, they have very efficient metabolisms).
Also, if you are using pellets, a fish who is used to flakes might find the hard, thick pellets tough to handle. If you soak these in a cup of water for a minute to soften them up they may go down easier and you can ween your fish onto them this way.
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