Weird Thing With My Friends Fish And The Tank Light

[captaincrash84]

Hi,

I was at my friends today as he was showing me his tank. He only has 3 Zebra Danios and He said its been up and running for 2 months now. When I was there he switched the aquarium light off and straight away the 3 fish swim to the top and linger up there until the light goes back on. He showed me a few time and without fail they always head up there and stay there until the light goes back on and they go about their business as usual. Is this normal? i'm new to fish and I thought it was very strange or is it normal?

Thanks if anyone can answer my curiosity.

 

[waterdrop]

Kind of hard to tell without actually being there to take in more variables. With there being only 3 danios, they are below their "minimum" shoaling number, which means their typical behaviours will be somewhat less predictable and they will likely be more stressed than average. Up at about 6 danios this would change in my experience.

When you have min shoaling numbers and above (and this is not to say danios shoal as strongly as most tetras, they don't) and when other tank conditions are good you will often see them weaving about the upper part of the tank, quite near the surface, at pretty high speeds and pretty much hanging as a group. This can change if watched throughout the day and they will often calm down and explore the bottom for food and generally other parts of the aquarium.

I suppose one might speculate that what you observed might have something to do with there being lots of insects above water bodies in the evenings and mornings when the sun angle is not direct, so these "minnows" might have a bit of evolutionary selection for greater feeding success at the surface when the light level diminishes. In contrast, another evolutionary pressure might have to do with the danios *being* food! It could be that they have had greater success hiding from larger lower swimming fish who want to eat them and who come farther up when there's less light by staying closer to floating plant coverage. If seeking floating plant cover up high for safety is an instinct, it would be practiced regardless of whether there's cover there or not.

Maybe they just like to sleep up high

~~waterdrop~~