How Tanks Are Made
Why a Water Tank Must Not Let Light Through
Algae need light to grow. A tank wall that blocks light gives them nothing to work with, which is why the middle layer of a Vee Tank is grey and opaque — and why an opaque wall does more for water quality than any additive.
5 min read
In short: algae are plants. They photosynthesise, and photosynthesis needs light. A tank wall that light cannot pass through removes the one input algae cannot do without — so the water stays clear because of how the tank is built, not because of anything added to it.
This is the least visible of the three jobs a tank wall does, and the one most often confused with the others. The outside of a tank is black to protect the plastic from the sun. Stopping light reaching the water is a separate requirement, and it is met in the middle of the wall.
The green water problem
Fill a clear bottle with water, leave it in the sun, and within a week or two it turns green. That is algae, and it needs three things to establish:
- Water — unavoidable, it is a water tank.
- Nutrients — trace amounts arrive with dust, roof runoff and the supply itself.
- Light — the energy source for photosynthesis.
You cannot remove the first. You can reduce the second with a first-flush diverter and a covered inlet, but not to zero. The third is the one you can shut off completely, and shutting it off completely is what an opaque wall does.
No light, no photosynthesis. No photosynthesis, no bloom. It is a simple mechanism and that is exactly why it is reliable — there is no dose to get right and nothing to wear out.
Translucent is not the same as opaque
This is where tanks differ, and where a buyer can be caught out.
A wall can be thick, strong, and still let light through. Hold a phone torch against the outside of a tank in a dark room and look from the inside — or, more practically, look at how the wall behaves against bright sunlight. If you can see a glow, light is getting to the water.
A dark exterior helps but does not settle it on its own; what matters is whether light makes it all the way through the full thickness of the wall. That is why an opaque layer inside the wall is worth having as a specific, separate feature rather than something assumed from the colour of the outside.
What the middle layer does
In a Vee Tank the middle layer is grey, for added strength and opaqueness. It sits between the black outer layer and the white inner one, and it does two jobs at once:
It blocks light. The grey pigment stops what the outer layer has not already absorbed, so the water sits in the dark whatever the sun is doing outside.
It carries the wall. It is the thickest part of the section and the part that resists the pressure of a full tank pushing outward — which, in a 10,000 litre tank, is ten tonnes of water leaning on the bottom of the wall.
Because Vee Tanks are blow moulded, the middle layer is formed in the same shot as the other two rather than bonded on afterwards. There is no interface between the layers that could separate.
About "anti-algae"
You will see tanks — including ours — described as anti-algae, and it is worth saying plainly what that does and does not mean.
In the overwhelming majority of cases, an anti-algae tank is a tank that blocks light. It is not an additive doing the work, and you should be sceptical of any tank claiming an anti-algal chemistry in a surface your drinking water touches. Keeping the light out is a better answer than dosing the wall, because it cannot leach, cannot be used up, and cannot stop working after five years.
So the honest version of the claim is the mechanical one: the wall is opaque, algae get no light, and algae therefore do not establish. That is a real property you can verify yourself with a torch, which is more than can be said for most additive claims.
What an opaque wall still will not do
An opaque wall stops algae. It does not stop everything:
- Sediment still settles out of the water and collects on the floor.
- Biofilm — a bacterial film rather than an algal one — can develop at the water line without any light at all.
- Contamination through the lid or inlet is unaffected by the wall. A badly fitted lid or an unscreened overflow lets in dust, leaves, insects and worse.
Which is why an opaque tank still needs cleaning on a schedule, and why the lid and the inlet screen deserve the same attention as the wall. The wall removes one problem completely. It does not remove maintenance.
How to check before you buy
- Look at the wall in strong sunlight. Any glow through it means light is reaching the water.
- Ask what makes it opaque. "There is an opaque layer in the wall" is a specific answer. "It is black" is not the same claim.
- Look at a cut section if one is available. A layered wall shows you its layers.
- Ask about the inner surface too — what it is made of matters as much as what the wall blocks.
More marks of a well-made tank are in what is the best quality water tank to buy.
The Vee Tank wall
Three layers, formed together in one shot:
- Premium outer layer, black, for UV protection
- Middle layer, grey, for added strength and opaqueness
- Inner layer, white virgin material, food grade and anti-algae
Six sizes from 1,000 to 10,000 litres, made in Kenya by Astral Industries Limited, with a 12-year warranty.
See the range, or ask us anything.
Sources for the general material science in this post: how light drives algae growth in stored water and opaque tank walls and algae inhibition.