The load is different from every other framing job

A machine frame carries a load that is known, concentrated and often dynamic. An aquarium carries a load that is spread evenly, never removed, and completely predictable — and that difference changes what the frame has to be.

Water weighs very close to 1 kg per litre, so the arithmetic is simple and the numbers are large. A tank holding 500 litres imposes half a tonne on its base before the glass, the substrate and the cabinet are counted. There is no impact factor and no dynamic allowance, but there is also no relief: the load is present every hour of every day for the life of the installation.

Two consequences follow. The first is that creep and long-term deflection matter more than yield strength, because the frame is loaded continuously rather than intermittently. The second is that the base must be continuous. Point support at four legs puts the glass panel into bending between supports, which is a failure mode the glass was not selected for.

Translating tank volume into a frame load

The conversion is worth doing explicitly because it is where aquarium projects are most often under-specified. Volume in litres gives the water mass almost directly; the glass adds a substantial further amount on larger tanks; substrate, rock and equipment add more.

Those design loads place most display aquaria squarely in the heavy-duty band of structural framing, and the larger ones beyond it. A 1000 litre installation is carrying well over a tonne, continuously, which is not a 20 series or light 40 series job and should not be specified as one.

The other number that matters is the span. A base rail that is supported only at its two ends over a 2 m tank length is a beam under a distributed load of a tonne, and its deflection is the number that determines whether the glass stays flat.

Why the base must be continuous

Glass in an aquarium is loaded in bending by water pressure, and it is supported along its bottom edge. If that edge sits on a continuous surface, the load transfers evenly. If it sits on a frame that touches it only at intervals, the panel spans between the touch points and the bending it was never sized for appears.

This is the single most common framing error in aquarium stands, and it is invisible until the tank is filled. The frame looks correct, the tank sits level, and the base panel is quietly carrying a load case nobody calculated.

The same logic applies to level. A frame that is out of level by a few millimetres puts a torsional load into the tank as the water settles to one side, and glass does not tolerate torsion. Levelling feet are not optional on this application; they are part of the load path.

Salt water: the two corrosion mechanisms

A fresh water display is a demanding environment. A salt water one is a chloride environment, and chlorides attack aluminium in a specific way that section size does nothing to address.

The first mechanism is pitting. Chloride ions penetrate the oxide film locally and start a pit that grows while the surrounding surface stays intact. 6063, the standard extrusion alloy, is more susceptible to this than 6061, which is why marine framing generally starts from 6061 even though it costs more and extrudes less easily.

The second mechanism is galvanic corrosion, and it is entirely a specification problem. Aluminium is anodic to stainless steel. Put a bare stainless bolt into bare aluminium, add an electrolyte — and salt water is an excellent one — and the aluminium corrodes to protect the bolt. The bolt looks pristine and the hole around it disappears.

Both mechanisms are addressed the same way: choose the alloy for the environment, specify the coating by thickness rather than by name, and isolate every dissimilar metal contact.

What to send with the enquiry

Aquarium framing is one of the applications where a supplier can be genuinely helpful early, because the load is fully determined by the tank dimensions and the environment is either fresh or salt. Four numbers and one word are enough to start.