Before panicking there are a few things you need to ask yourself. When ammonia builds up too much your betta could begin suffering from ammonia poisoning, which can quickly become fatal. Also if another chemical has accidentally been introduced into your tank then it could also be damaging your betta. On the other hand, if you have a filter you may notice your betta stays at the bottom of his tank as well. And it may be because the current coming from your filter is too strong.
PLEASE HELP - BETTA FISH STAYING AT BOTTOM OF TANK
fish - How can I help my betta who is laying at the bottom of his tank? - Pets Stack Exchange
My fish just exploded his guts are all over the tank. I am very sorry for what happened. It is very difficult to guess what happened at this stage. I would recommend to perform a gradual but generous water change to eliminate Blahh insides from the water Separate the fish out in to a small tank or bowl of possible.
11 Reasons (+ Tips) Why Your Betta Fish Lays at the Bottom of the Tank
Are they sick? Although cats often hide the fact that they are sick, their symptoms are usually fairly clear. Instead, I make sure that I keep a good amount of his old water to make sure there are enough nitrates to start his cycle. I would speak to a fish store clerk to see if they have similar bacteria in their store.
If that's actually the size of tank, its too small. While they do 'rest' a lot, bettas are actually fairly active fish. Watch them sometime in a big planted tank. In a small tank like that movement is restricted - especially laterally.