Sunlight and daylight

Sunlight and daylight. Even the most "natural" aquarium is likely to depend on modern fluorescent or LED lights. In fact you are always gravely warned never to set up a tank where direct sunlight can strike it. Indeed, long hours of direct sunlight could foster rampant algae, and in summer the sun's heat might raise the water temperature higher than most tropical fish can withstand. But Diana Walstad places her tanks where sunlight will strike them for several hours a day; she depends on vigorous plant growth and minimal levels of iron to keep a cap on algal growth. You can read about her techniques in Ecology of the Planted Aquarium, 1999, a book on the short list of essential fish-keeping books.
 
But even if you merely place a tank in an east- or northeast-facing window, where the early sun will throw stray beams into it, you may be able to use sunlight as a natural spawning cue that is irresistible for some barbs and danios and tetras. No artificial light, no matter how brilliant, can equal the power of sunlight: ask a reefkeeper about those vaunted "glitter lines" produced by LEDs. The authentic "full spectrum" of sunlight brings out glints of iridescent sheen. It transforms Pearl Danios or Rams, which give back violet and green opalescent highlights you never see under artificial light. As for sunlight problems: adjustable window blinds easily control unwanted sun. Additional seasonal protection could be afforded by a sheet of cardboard taped to the window-facing tank glass. A thin sheet of styrofoam or foam core would offer heat insulation too.
 
And black duct tape shields the gravel from unwanted sunlight in the lower anaerobic zones, where some esoteric kinds of bacterial photosynthesis are dis-commended. When iron in the Flourite is exposed to sunlight, it can be released into the interstitial water as soluble iron, according to Diana Walstad. If the iron is not scavenged by rooted plants in a well-developed rootzone, it will diffuse out into the water column as surely as a drop of ink will spread in a still glass of water. At large in the tank water, the iron will become available to algae.
 
Right now I have a densely planted 10-gallon that catches shafts of fairly hot late afternoon summer sun for an hour or more. Not an ideal time of day. But in summer this tank often has the clearest water of any of my tanks. All my west-facing windows have adjustable blinds. Doesn't everyone control the sunlight coming into their place?