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I just recently set up a 5 gallon betta tank. I had it running for about two weeks with a male betta in it. Everything was going great, he was eating, swimming about, curious, and happy. However, as soon as I added 5 pygmy corys he has completely done a 180. He hides ALL the time and rarely comes out. He just sits on top of the heater or in back of a plant. The corys are doing fine, active and eating. But my betta is acting sort of depressed. nNyone have any ideas?

What are your water parameters?
Since you added 5 fish at once you put a huge burden on the biofilter. A minicycle could be causing him to act differently.
I wouldn't expect a betta to be bothered by corys, but bettas can be weird at times. Sometimes they're fine with tankmates sometimes there not.

Did you increase water flow at all after adding the corys? Most bettas don't like much water movement. I say most, though I've had one that simply loved playing in the bubble wand . You might try adding a few more plants, usually they like a little bit of a hiding spot but being surface dwellers hardscaping doesn't do them much good.

Has the temperature stayed steady?
I had a heater go down on a tank and hadn't realized and till I noticed the betta acting rather sluggish sleeping on top of a plant. He stayed like that even after the heater was replaced, but he would perk up at feeding time.

Those are the only things I can think of, short of moving the corys carefully to another tank for a while to see if that's what is causing his stress.

^_^