Hi Fks!
I have a male betta in my 10gallon which is fully cycled. Water temp is always around 26-28C. pH is 7
I've had him for a week and he was doing fine until this morning. He seems to be a little sad (not active and swim around hunting the mosquito larvae I gave him, hiding in plants). Other than being inactive and doesn't want to eat, he has no bad physical sign.
I feed him with mosquito larvae (both live and frozen), flake fish food

Anyone know what's wrong with him and what should I do?

Things to think about - has he got enough plant cover for security, is the light too bright, perhaps turn up the heater a fraction to 29. Maybe you are expecting him to eat too much, they are reslly not very big fish when you allow for all the fancy finnage. And many bettas despise flake!
Sorry no real solution, let us know how he goes ...

About plants and light... he has enough plants to hide so well that I can't even spot him until he decides to show up, light is pretty low. I feed him flake rarely too only about once a week, his main food is mosquito larvae.
He still seem to be sad :( I told my friend about my fish and he joke that may be my fish needed a girlfriend. This reminds me that I saw 1 of my female has big and yellow belly and the white stuff. I added her to the tank but after 2 hours, the male still chasing her crazily which made her keep hiding from him.... I had to remove the female. Next morning he becomes like this :(

Hi nathan

Do you just want to check that pH as your profile states a pH of 8.5. Are you using remineralised RO to soften it and to make it more acidic? It could well be the pH. I've only ever kept Betta in about 6.6.

What size tank is he in? Other fish? They may well be crowding his territory and stressing him.

Male fighters are highly territorial towards all other Betta, including the ladies, so don't try adding any more The males will have a given territory and woe behold anything coming into it, even other fish that look remotely similar or who have bright colours. This causes terrible amounts of stress for all fish concerned. A single male is far happier in own tank and FK recommends 45 litres min.

Betta’s are harem spawners, not pair bonding at all, one of the forum members here used to have a harem tank and she stocked 1 male to 2 (possibly 3 females) only in a ‘big’ 4 foot tank species only tank. If the females enter the male's territory, they are expected to breed. If unreceptive, they'll be aggressively evicted from the territory and often fish in too small tanks, are commonly seriously injured and sometimes killed.

It can be carnage in too small a tank. The male will flare at females to try and lure them, but he will also flare to warn them, prior to an attack. The other absolutely crucial thing is lots of really dense planting to block off the line of sight and give respite to the females. To keep a harem set up you need a long tank and 4 foot minimum is recommended. Length is more important than volume in this scenario.

Hi and thanks for the information!
yes, the tap water in my area is 8.5 but water in the tank is always kept at 7 or lower. He's alone in a 50 x30 x40cm, densely planted but still lots of space for swimming, sponge filter. He still hasn't eaten any mosquito larvae in the tank, I can still see those larvae swimming and all he does is lay there do nothing!
I keep the female bettas in a different tank, the females seems to be very well. I just added a female to his tank to clean up the larvae before they turn into mosquito.... the female run around hunt larvaes down but the male seems to be doesn't care.
I'm thinking about doing a 25% water change

This is the third he has been like this... What's going on with him?? I really worried, he doesn't even eat!

Day 4.... nothing has improved, I can't figure out what's wrong with him... if this keep going, he will die for sure :(

This is it, can't save him now :(
I can't tell what's wrong with him but he finally show some phyusical sympton... which is too late now...
If anyone know what's the problem, please tell me. It looks like the lost some flesh..
pH is between 7-7.5
temp 27

How long have you had him? Can you post pic? To do that resize first and use the "Attach file" button at the botom of a new post, only 1 pic per post.

How are you reducing the pH? Do you know how hard the water is?

I don't know about the water hardness but I guess it's not very hard as the tetras in my other tank use the same source of water. I reduce pH by some kind of liquid the shop sold me, it's mainly use for raising discus fish but they say it works fine with others as long as I reduce only 0.5 per day so it won't shock the fish. It works fine with tetras too (I tried)
here's the pic
http://www.flickr.com/photos/75680215@N08/6794612334/