Linking water tanks together is an easy way to increase your water storage capacity without significant plumbing work. 

Better Tank Prices
A primary reason for linking multiple water tanks, is that often with water tanks there is an ideal price point per litre. The price point on these can often work out better than smaller or larger tank options.

Lower Maintenance Costs
Very large steel tanks can be quite expensive, as well as difficult to setup. There is also a cost to maintaining your rainwater storage system over time. It will be much less expensive to fix and/or replace one poly tank of say four 22,700 litres in a series, then it would be to replace a 100,000 litre steel tank. Linings also need fixing and/or replacing over time, and this can be quite costly with larger tanks.

Maximise Capture of Rainwater Harvestable
Another good reason is where you have many rooftops or perhaps good locations at more than one downpipe. A better option to maximise storing the high volume of water you can potentially harvesting, is to link several tanks together and make use of a wet water system (downpipes lead to an underground piping system to your rainwater storage location).

Expandable Water Storage
If you are unsure of how many water tanks you will need, or perhaps you might need to store more water in the future, then a multiple water tank setup obviously means you can add more tanks as needed. You just need to ensure that you have adequate space on your property for installing another rainwater tank.

Water Storage Fault Tolerance
Should a water tank become damaged, fail and leak, then obvious in a single tank setup your only option is to lose your water. Your tank will likely need to be completely drained before the problem can be fixed, or when you install a replacement tank.

A more modular water storage system, like a multiple water tank setup, means should one tank spring a leak then you don’t stand to lose all your rainwater. That tank can be isolated and/or have water pumped into your other tanks before it is fixed and/or replaced.

In summary:

• Multiple tanks can be more cost-effective than buying one very large tank
• Multiple tanks can have lower maintenance and replacement costs than one very large tank
• Multiple tanks provide a convenient way to maximise rainwater harvesting with multiple roofs or many downpipes
• Water storage can be expanded with extra tanks as needed
• Multiple water tanks provide higher fault tolerance due to modular nature (if one tank is damaged, it can be fixed or another ordered while the other tank/s still harvest rainwater)

Linking Tanks to Fill Evenly
To link tanks together so that they both evenly fill is accomplished by linking the tanks together near the bottom at their sides with piping. As your first tank fills up and water reaches the height of your link, rainwater will flow into your second tank until it attains the same height. At this point, both tanks will evenly fill up until full.

A benefit to this setup means you only need to tap off and/or pump water from your first tank (or tank that is lowest). Equally only one overflow is required in such a setup. It is important with tanks of different heights, that the tops (overflow) be aligned with each other. This normally means raising the smaller tank up on a higher foundation, or digging out beneath your taller tank before establishing its foundations.

It is important to include isolation valves if using this method, so that if one tank springs a leak, it will not drain all your tanks of their stored rainwater.

Linking Tanks to Fill One at a Time
Tanks linked in series through their overflow will result in your first tank needing to fill up before rainwater flows into the next. The last tank in the series is setup with an overflow to expel excess water to your storm water drainage as per a normal single tank setup.

To use water in all your tanks, each tank needs tapping and/or to be connected via piping at your desirable outlet level on their side. All tanks are piped from their outlet to underground piping that leads to your pump.

Again, you should have isolation valves at your tank outlets so the release of water in a tank can be controlled. This setup while more complicated, also can provide you with the best of both worlds should you desire tanks to fill up at the same time, simply open all valves. If individually, one after the other, close all except the first and open the next in the series as needed. This can be performed manually, or auto-switching can be setup to refill your storage tank should it drop below a certain level.

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Types of Bases Suitable for Polycon Water Tanks

For polyethylene water tanks, the most suitable base must be a solid, flat surface. Your manufacturer will be able to provide recommendations, but generally the best options are bases made of concrete or boxed compacted crusher dust.

“Crusher dust” is available from nursery suppliers and is a mix of small crushed rock and fines. When compacted it binds together tightly creating a strong, stable surface for placing your tank. The crusher dust must be boxed in with some sort of retaining structure (e.g. retaining beams, bricks) to prevent it washing away over time.

Concrete is the best base for a slimline tank because the shape of the tank (tall and narrow) means a risk of the tank tipping over if not on a solid base.

Tank stands are another option for the tank base, but must be strong enough to hold the weight of the tank when full. Use hardwood decking with gaps no greater than 10mm.

Regardless of the materials you choose, the ground must be compacted before laying the concrete or crusher dust, and the base must be carefully laid and finished to ensure it is flat and level.

Given the importance of the finished base, it may be worth considering getting help from someone experienced in concreting to ensure you get the base right. Alternatively, if you’re using crusher dust it is vital you use appropriate compacting equipment to get the best result.

Types of Bases to Avoid for Polyethyelene Water Tanks

An unsound base or a base which erodes may void your poly tank manufacturer’s warranty.
Take care to avoid the following:

• Rocky and uneven ground with little preparation causes under-mining of the base of the tank and sharp objects can protrude through.
• Bricks or timber sleepers, as these can move or erode over time.
• Corrugated iron decking should never be used as it is unstable and may stress the tank and cause the tank to fail.

You should also take the time to ensure there are no tree roots that are likely to intrude on your tank’s base area over time.

Do I need to leave space around my tank base?
Yes, a larger base provides stronger foundations and reduces the risk of erosion and the foundation slipping. If your foundations are poor, then your tank warranty can be compromised and/or your tank ruined under the weight of a full tank of water.

How much space should you leave?

• If you are installing a polyethylene water tank, then a 100mm space around each side of the tank is normally sufficient.
• For round poly tanks, the base should be at least 500mm greater than the tank diameter.

Most importantly, always follow your manufacturers’ instructions to ensure you meet the requirements of the warranty.

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