Fish behaviour: water flow vs. lighting – pt 2

Following up on observations of changing fish behaviour when either lighting or flow rates are changed, here we take a look at what happens when the lights turn off.

The video shown starts after the Kessil A80 light has been on for nearly 6 hours, the typical photoperiod for the current setup.  The tetras and barbs are towards the bottom right portion of the tank, as they have been for almost the whole day.  Once the lights turn off (automatically) they become more adventuresome, and expand their range to the upper reaches.  To see whether this was a temporary reaction to the change in lighting the video also skips ahead to 30′ after the lights have turned off, and the fish are behaving in about the same way, suggesting is it the state of the lighting rather than the change in lighting that is driving their behaviour.

More thoughts after the break…

Lights turn off at 0:30
30 minute time jump to low flow at 1:25
High speed flow at 1:57

After things have stabilised with the lights off (and mind you the room is still reasonably bright since it’s still daylight outside and the room lights are on) we can take a look at the difference changing flow patterns have, which seems to be not much.  The powerhead is set-up to alternate between 2′ at the lowest setting and 2′ at the next-to-lowest setting, repetitively, all day long – Kessil calls this “gyre mode”.  In this small tank even the next-to-lowest setting pushes things around pretty good, again easily seen with the ludwigia.  The plants blow around a lot more on the higher flow speed, but the fish just seem to keep on keeping on.

So it seems it is the overall state of the lighting that is mostly determining where the tetras and barbs hang out in this set-up:  they’re down in the lower reaches when then lights are on, and more at the middle/top above the lobelia when the lights are off.  When the lights turn on the change in their behaviour happens right away and whilst the reciprocal change the other way when the light turn off happens with a little less immediacy, the change to a stable form of “lights off” behaviour pattern still largely happens within the first minute of the change.  They’ll stay in this upper part of the tank during and after their fed, and generally over the whole night, until the main aquarium light comes on again the next morning.