10-15-2009, 01:45 PM
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This is a good question, as many never consider the effect of light on fish. Many of the fish we keep in aquaria come from dimly-lit waters that receive no direct sunlight but only diffused daylight due to overhanging vegetation and/or aquatic vegetation. In both South America and SE Asia the rivers and streams regularly flood the surrounding forest for up to six months, and it is during this period that the fish naturally spawn because food is plentiful. But the waters are dark, either due to low light reaching them or the naturally dark, tannin-stained waters known as blackwater.
Such fish will always feel more "at home" in less light. And with less light, the darkness of the substrate plays another important role. Interestingly, fish will exhibit their best colouration and behaviours over dark substrates and less light, and aside from others saying this I have personal experience frommy own aquaria. In bright light, as MM pointed out, such fish will constantly look for hiding places, and if they cannot find them will be stressed. And ill health follows, perhaps disease or simply a shorter life than they would otherwise have. The fact that some of these fish will not spawn except in minimal light such as they would have in nature is suggestive; the degree of light clearly has a bearing even on mature fish. Some writers have commented that cardinal tetras and similar forest fish actually have a light phobia.
As a planted aquarium hobbyist, my aim has always been to provide the least amount of overhead lighting, just sufficient for the needs of the plants. The majority of fish one keeps in planted aquaria are forest fish requiring little if any direct light aside from our desire to observe them. And in both cases, fish and plants, they require 10 hours of total darkness.