In transitioning from a low tech to high tech tank I noticed a significant change in the behaviour of the fish* where they used to swim mostly at the top/middle regions of the tank under low tech conditions but then spent a lot of time at the bottom with the high tech conditions.  At the time I attributed this change in behaviour to the increased water flow from installation of a powerhead, but now I’m not so sure.

Embedded is a video that starts with the tank in a lights off, low flow state.  This is 4 hours after the lights have turned off for the day at which time the fish also got fed so they’re not behaving in a hungry mode anymore here.  After the first minute the powerhead will automatically change from low flow mode to higher flow, and after the 2 minute mark I’m going to manually turn on the main tank light.

Observations below…

 * The otocinclus catfish and shrimp don’t seem to care about either flow or lighting – they pretty much always behave the same way regardless.

High speed flow starts at 1:02
Lights come on at 2:20

The first observation is that increasing the water flow speed doesn’t really change the fish behaviour much.  You can see the effect of the significantly increased flow in the motion of the ludwigia but the fish are pretty unperturbed.  There might even be some increased “playfulness” behaviour in the current.  The fish definitely don’t move out of the middle regions of the tank.

When the light comes on however, the effect on the fish is rapid and dramatic:  they school down to the bottom of the tank and hide underneath the lobelia.  I’ve noticed they do this in the morning when the lights come on automatically as well.  As the day progresses they’ll come out from under the plants but will still generally remain in the lower reaches of the tank.  Towards the early afternoon when light have been on for a number of hours they’ll progressively become a bit more adventurous (possibly anticipating lights off and feeding time) but not to the extent that they move to the middle zone.  After the light automatically turns off in the late afternoon they will pretty aggressively enter the upper reaches and come over to see people that come near – “Time for food?  Food for fish?  You feed fish now?”

So now on rethinking things, it may have been the switch to much higher intensity lighting mostly driving the changed behaviour in a high tech vs. low tech tank setup.  That and possibily the significantly greater plant growth, after all it’s easier to hide in the plants when there are actualy some sizeable plants around!

1 thought on “Fish behaviour: water flow vs. lighting

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