Sunlight is far stronger than any aquarium light
An aquarium light is deliberately modest: enough for plants, limited to a set number of hours, and the same every day. Sunlight through a window is none of those things. Even on an overcast morning it can outshine a tank light, and on a clear day it delivers intense, full-spectrum light for as long as the sun crosses that window. Everything photosynthetic in the tank responds at once. In a newly set up tank with nutrients to spare, that response is the familiar green takeover: algae filming the glass, coating plants, and in bad cases tinting the whole water column green. The fish did nothing wrong and the filter cannot fix it, because the cause is arriving through the window every day.
The greenhouse problem: temperature swings
Algae is the visible damage; heat is the dangerous one. Water absorbs sunlight and the glass slows the heat escaping, so a tank in a sunbeam warms like a parked car. Small tanks, with little water volume, heat fastest — a nano tank on a sunny sill can climb several degrees in an afternoon and drop back through the night. Fish are cold-blooded and built for the slow, small changes of a river or pond, not a daily spike and crash. Repeated swings suppress their immune systems and stress shy species into permanent hiding. Warm water also holds less dissolved oxygen at exactly the moment the algae and bacteria in a sunlit tank are consuming more of it, so a hot, still night after a sunny day is when oxygen crashes happen.
Why indirect light is fine and sun is not
None of this means the tank needs a dark room. Ordinary room light and bright indirect daylight are harmless and help you enjoy and inspect the fish. The problem is specifically a direct beam striking the glass: concentrated, moving across the tank through the day, and strongest exactly when the room is warmest. A tank a few feet back from the same window, lit by bounce and ambience, gets the viewing pleasure with none of the runaway light or heat. Plants inside the tank should get their light from the aquarium light above them, on a timer, so their hours are fixed whether the day outside is blazing or grey.
If the tank is already in the sun
Sometimes the only feasible wall catches the afternoon sun, and moving a filled tank is a serious job. Blunt the beam first: a blind or curtain closed during the sunny hours, a solid background panel on the sun-facing side of the glass, and floating plants to shade the surface all cut the dose sharply. Watch the thermometer at the end of a sunny day for a week, since that reading tells you whether the fix is enough. If the swing is still large, the tank genuinely has to move, and it is worth doing properly — fish and most of the water into buckets, tank emptied before lifting — rather than living with a daily stress cycle.
Keeping the solution in place
Sun damage usually returns through kindness: someone opens the curtains for the plants on the windowsill, or tidies the blind away. Once the tank's spot and its lighting routine are written into its KinsLe Family Sharing profile, the whole household knows why that blind stays shut at midday and the tank stays out of the sunbeam.