why is my fish acting strange???

Question

Ok I have a 75 gallon saltwater fish tank and i fish got to crab then i added i sargent major, and a blue green chomie and they were fine to got along well and at night when i turned down the light then even sleep in the same little hiding spot but yesterday i added two chocolate chip starfish and 1 clown fish and two of these snails that stay under the sand. evey thing was going well all day then i turned the lights out for about 1-2 hours and wheni came to check on them my clown fish was layin in the corner on its side and by sargent major was al most completely black and my chomie was hiding from him cuz for some reason the sargent major started bitting him after like 30 mins every thing went back to normal and they seem to be getting alone just fine but i think that my turning the light out had sone thing todo with it so im scared to turn it back out . does anyone know what could have caused this. PLEASE HELP!!!!!!

Answer 1

What's happening is that the "older" fish have all set up their territories and the clownfish was a newcomer and didn't have anywhere of it's own - the other fish had already claimed all of the tank area. All three of these fish (the clown, sergeant major, and chromis) are all from the same family of fish, and they will be aggressive toward one another (except for the chromis which is a peaceful species).

The sergeant major will probably not get along in the long term - yours are probably small yet, but by the time it is full grown (at 6 inches) it may kill the clown, or stress it so much it may not come out to eat.

You might try taking the sergeant major out of the tank and rearranging everything in the tank so none of it looks the same. After about 1/2 hour, put the sergeant major back. This will give the chromis and clown time to get used to the new setup, and the old territories won't exist any more, so the sergeant major will have to create a new one. This will give all three fish a chance to have their own space. There's no guarantee this will always work with saltwater fish, but it seems to be a good strategy for territorial freshwater species. You'll just have to keep an eye on them. If they still fisht, you may need to separate them permanently with a second tank, or retuen one to the store.

It's normal for fish to change color when the lights are out. This helps keep predators from being able to see them as easily. Some will get darker, others will get pale.

Answer 2

He was taking the opportunity of nighttime to feed, and he sees the clown fish as prey.

Not much you can do about this except separate them.