Where the tank goes: sunlight, heat sources, floor load and balconies

Quick answer

Where an aquarium stands does more to determine whether the fish stay healthy than almost anything you put inside it. A tank in front of a sunny window is a tank with a daily temperature swing, an algae problem and, in summer, an oxygen problem.

The second half of the decision is structural. A filled aquarium is a very heavy object standing on a small footprint, and tanks fail at the seams when they are not level or not properly supported.

Bright room, no direct sun on the glass, level surface, and space to work on all sides.

• Direct sunlight causes temperature swings and algae blooms, and neither is fixed by better filtration.
• Warm water holds less dissolved oxygen, so the hottest part of the day is when fish are most likely to gasp.
• A litre of water weighs a kilogram, so a hundred litre tank exceeds a hundred kilograms before you add glass, substrate and rock.
• An aquarium that is not level puts uneven stress on the silicone seams, and seams fail suddenly.
• Small tanks swing further and faster than large ones, so a nano tank needs a better position, not a worse one.

Decide in 60 seconds

Why sunlight is the main problem

A glass box in direct sun behaves like a small greenhouse. Light energy enters, is absorbed by substrate, decor and water, and becomes heat. The tank warms through the afternoon and cools overnight, and it does this every clear day.

The fish experience that as a daily temperature cycle. Fish are ectotherms, so their metabolic rate follows the water temperature directly, and a repeated swing is a repeated physiological demand. Sustained instability suppresses immune function, and the practical result is that parasites and bacterial infections which the fish had been coping with begin to take hold. Outbreaks of white spot frequently follow a week of unstable temperature rather than the arrival of anything new.

Heat brings a second, more immediate problem. The amount of oxygen that water can hold falls as the water warms. At the same time, warm fish have a higher metabolic rate and need more oxygen. Those two curves move in opposite directions, which is why fish gasp at the surface on hot afternoons. Adding an airstone helps because it improves gas exchange at the surface, but it cannot raise the ceiling that temperature has lowered.

Then there is algae. Algae need light and nutrients. Sunlight supplies far more light energy than any aquarium lamp, for free, for hours a day, and no amount of scrubbing or filtration keeps up with it. Green water and heavy algae on the glass in a tank by a window are a lighting problem, not a cleaning problem.

If the tank cannot be moved, the answer is to block the light rather than to fight the consequences: blackout film or a solid backing on the window-facing side, a blind that is actually used, or a solid background applied to the glass.

Heat sources, cold sources and where they collide

Radiators are usually under windows, which means the classic bad position is bad twice over: sun in the afternoon, radiator heat in the evening, cold glass overnight.

An aquarium heater can only add heat. It has no ability to cool, so any external heat gain pushes the tank above the set temperature and stays there until the room cools. A thermostat set correctly does nothing about a tank being warmed from outside.

Cold works differently but is equally disruptive. A tank against a poorly insulated external wall, or beside a door that opens to the outside, loses heat continuously. The heater runs almost constantly to compensate, which shortens its life and means that when it fails the temperature falls fast.

Underfloor heating deserves a specific mention. A tank standing on a heated floor is warmed from below in a way that is very hard to control, and the tank stand may also be affected.

Kitchens are worth avoiding for a different reason: cooking aerosols, cleaning sprays and aerosol products all reach the water surface, and fish are highly sensitive to surfactants and to many household chemicals.

Record the tank's highest and lowest temperature over a full week in the intended position before you commit to it. The swing matters more than the average.

Weight, floors and levelness

Water weighs a kilogram per litre. Add glass, a stand, substrate, rocks and equipment and a modest hundred litre aquarium is well over a hundred kilograms, concentrated over a footprint smaller than a desk.

On a solid ground floor this is rarely a problem. On a suspended timber floor it deserves thought, and the larger the tank the more thought. As a general principle, a tank is better placed across the direction of the floor joists rather than along the length of a single one, and better placed near a supporting wall than in the middle of a span. For anything substantial, particularly on an upper floor or in an older building, take advice from a builder or structural engineer rather than guessing. This is one of the few aquarium questions with a genuinely structural answer.

Levelness is a separate issue and it causes more tank failures than floor loading does. An aquarium is a box of glass panels held together by silicone under constant outward pressure. If one corner is unsupported or the surface is not flat, the stress concentrates at a seam, and seams do not leak gradually as often as people expect. They fail.

Check the stand with a spirit level in both directions, before filling and again after. Use a levelling mat under a rimless tank so that small irregularities in the surface do not become point loads. Never shim only one corner and consider it solved.

Leave working room. A tank that is accessible from the front only is one you will maintain badly, and maintenance is the whole game in fishkeeping. Space at the back for cables and hoses, and enough headroom above to get a net and a hand in, are practical requirements rather than luxuries.

Electrical safety at the tank

Fit a drip loop on every cable: let the cable hang below the socket level before rising to it, so that water running down a cable drips off rather than entering the socket.

Put the sockets to the side of the tank rather than directly underneath it, and use a residual current device.

Keep any external filter, its taps and its hoses where a leak would be seen rather than hidden behind furniture.

Have a plan for a power cut. In cold weather, insulating the tank with blankets or a duvet and keeping the lid closed retains heat for a surprisingly long time, and a battery-operated air pump maintains surface agitation and gas exchange. In hot weather the priority reverses: open the lid, increase surface movement, and keep the room dark.

Balconies, windowsills and outdoor tubs

A balcony changes the risk profile completely, and the questions are different from those indoors.

Structural limits. Balconies have load limits, and those limits are often lower than the floor inside. In flats there may also be rules about what may be placed on a balcony. Check both before you plan anything.

Temperature extremes. Outdoor water is at the mercy of the weather. Shallow containers heat rapidly in summer sun and can freeze through in winter, and a small volume does both faster. The volume that survives outdoors is far larger than most people assume, and depth matters more than surface area for stability.

Predators. An outdoor container is a food bowl. Herons, corvids, cats and, depending on where you live, other wildlife will take fish from shallow water. Netting or a mesh cover is not optional, and it needs to be secured rather than laid over the top.

Chemical drift. Balconies collect what falls from above: cleaning water, paint, pesticide spray from other balconies, and run-off from treated surfaces. Fish are extremely sensitive to surfactants and to many garden chemicals.

Debris. Leaves, blossom and dust decay in the water, consume oxygen and release ammonia.

Escape. Fish jump. An open outdoor container in a position where a fish landing on a hard surface cannot get back is an avoidable loss, and it is a common one.

A windowsill is the worst of both worlds: full sun, a cold pane at night, no room for equipment, and no realistic way to level the surface. Small tanks on windowsills are a common beginner arrangement and a common early failure.

What changes with the type of setup

Nano tanks. Least thermal mass, largest and fastest swings. These need the most stable position, not the least, which is the opposite of how they are usually placed.

Unheated coldwater tanks. These have no thermostat to defend them at all, so room position is the only temperature control there is.

Planted tanks. Sunlight wrecks the lighting balance you are trying to achieve, and algae outcompete plants under uncontrolled light.

Marine and reef systems. Less tolerant of temperature variation and often running lights and pumps that add heat themselves, so ambient room temperature and ventilation around the cabinet matter more.

Large tanks. Structural questions become real, and so does the consequence of failure. A large tank emptying into a room is a serious incident.

Plan for the equipment before the tank is full. Cable runs, hose routes, a drip loop and room to work all get harder once there is water in it.

What never to do

Do not place a tank in direct sunlight because sunlight seems natural. Sun through glass on a fixed volume of water is not comparable to sun on a river.

Do not stand an aquarium on domestic furniture that is not rated for the load.

Do not fill a tank without checking it is level, and do not level it by wedging a single corner.

Do not put a heater in a tank without also having a reliable thermometer. A failed thermostat can cook a tank, and the only warning is the temperature reading.

Do not run cables straight up into a socket. Fit a drip loop.

Do not site a tank in a kitchen next to a hob or where aerosols are used.

Do not place any outdoor container without a secure predator cover.

Do not top up an outdoor tub or pond with untreated tap water without accounting for chlorine, and do not let run-off from a treated balcony surface enter it.

Do not move a partially filled tank. Glass tanks are designed to be supported evenly and filled in place; moving one with water in it stresses the seams and frequently breaks them.

When to get help

Get advice from a builder or structural engineer before installing a large aquarium on a suspended floor, on an upper storey, or on a balcony.

Get advice from an experienced aquatic retailer or an aquatic veterinarian if fish are gasping at the surface, if you are seeing repeated disease outbreaks that follow warm spells, or if algae are uncontrollable despite reasonable maintenance, because all three point back at position.

Act immediately, without waiting for advice, if the tank temperature has risen sharply, if fish are at the surface gulping, or if you find water on the floor around the stand. A weeping seam becomes a failed seam without much warning.

Stable temperature, controlled light and easy access are what let a tank settle into being uneventful.

Bring, or have ready, the tank volume and dimensions, the logged maximum and minimum temperatures, a description of the position including which way the window faces, the water test results for ammonia, nitrite and nitrate, the stocking list, and how long the tank has been running. Most persistent problems in a poorly positioned tank are diagnosed from the temperature log rather than from the water chemistry.