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Connecting two or more water tanks is a practical way to increase storage without replacing your existing tank. The safest setup depends on whether you want the tanks to act like one large reservoir or fill one after another.
For most rainwater systems, the simplest approach is to connect tanks near the bottom with a balance pipe. Water then moves between the tanks and settles at roughly the same level. Each tank should also have a suitable overflow, stable base, and fittings made for the tank material and connection size.
How Water Tanks Can Be Connected
There are two common ways to connect water tanks:
- Bottom connection: A pipe joins the tanks near their bases so they fill and drain together.
- Top or overflow connection: The first tank fills before excess water flows into the next tank.
Neither method is best for every system. The right choice depends on how you want the tanks to fill, where your outlet or pump is located, and whether the tanks are at the same height.
Method 1: Connect Tanks at the Bottom
A bottom connection makes several tanks behave much like one larger tank.
Each tank has an outlet or tank fitting near the bottom. A pipe connects these outlets.
As water enters one tank, pressure pushes water through the connecting pipe and into the other tank. Given enough time and adequate pipe capacity, the water surfaces settle at about the same height.
Typical layout
A basic two-tank arrangement looks like this:
Tank 1 → bottom fitting → connecting pipe → bottom fitting → Tank 2
You may also add isolation valves between the tanks so one tank can be disconnected for cleaning or repair.
What is a bulkhead fitting?
A bulkhead fitting is a watertight fitting installed through the wall of a tank. It creates a threaded or slip connection where a pipe, valve, or hose can be attached.
Some tanks already have molded outlets. If they do, using those outlets is often simpler than drilling new holes.
Do not drill a tank unless the tank manufacturer allows it and you know which fittings and sealing method are suitable for its material and wall thickness.
Tanks should be on the same level
Bottom-connected tanks work best when:
- The tank bases are at approximately the same elevation.
- The tanks are similar in height.
- The connecting pipe stays open and unrestricted.
- Each tank is properly supported.
A tank sitting noticeably lower than another can carry a greater share of the stored water. It may also overflow before the higher tank appears full.
Do not try to correct an uneven foundation by supporting a full tank with loose blocks or makeshift shims. Water is heavy, and large tanks need a stable base designed for their loaded weight.
Choosing the Connecting Pipe Size
The balance pipe must move water between tanks quickly enough for the incoming flow.
A very small pipe may technically connect the tanks but still cause problems during heavy rain. The first tank can rise faster than water can pass into the second tank.
The required pipe size depends on:
- How quickly rainwater enters the tanks
- The number of tanks
- The size of the tank connections
- Pipe length
- Bends and restrictions
- Height difference between tanks
- How quickly water is withdrawn
Use fittings and pipe that are compatible with the existing tank outlets rather than reducing everything to a small garden-hose connection simply because it is convenient.
For a large roof or high-flow collection system, have the expected inflow and connection capacity checked rather than assuming a small balance line will keep up.
Add Isolation Valves
An isolation valve between tanks can make maintenance much easier.
For example, you might arrange the system as:
Tank → valve → balance pipe → valve → Tank
Closing the valves allows one tank or part of the connecting pipe to be serviced without automatically draining every tank.
Place valves where you can reach them after the tanks are installed. A valve hidden behind a large tank may be almost impossible to operate later.
Make sure valves are left in the correct position during normal operation. Accidentally closing a balance valve can cause one tank to fill and overflow while another remains partly empty.
Method 2: Connect Tanks Using Their Overflows
Another option is to let one tank fill before sending water to the next.
In this arrangement, the overflow from Tank 1 runs into Tank 2.
A simplified layout is:
Downpipe → Tank 1 → overflow pipe → Tank 2
When Tank 1 reaches its overflow level, additional water enters Tank 2.
This is sometimes called a cascading or sequential setup.
Advantages of an overflow connection
It can be useful when:
- You cannot easily connect the tanks at the bottom.
- The tanks are arranged in a line.
- You want the first tank to fill before the second.
- The tanks are not intended to drain as one reservoir.
Limitations
The tanks do not automatically equalize.
Tank 1 may remain full while Tank 2 is partly full. If you draw water only from Tank 1, water stored in Tank 2 will not automatically move backward unless another connection is provided.
The overflow pipe also needs enough capacity for the incoming rainwater. Otherwise Tank 1 may overflow somewhere other than through the intended pipe.
Should Each Tank Have Its Own Overflow?
Providing a safe overflow path for the system is essential.
The final tank normally needs an overflow that can carry excess water away from:
- Tank foundations
- House foundations
- Walkways
- Septic areas where additional water could cause problems
- Places where erosion or flooding could occur
Depending on the layout, individual tanks may also require overflow provisions.
Keep overflow openings protected against pests where appropriate without using screening so restrictive that debris can easily block the overflow.
A tank must also be able to admit and release air as its water level changes. Do not turn a normal atmospheric rainwater tank into a sealed pressure vessel.
Connecting Several Tanks With a Manifold
For three or more tanks, a manifold can make the layout cleaner.
For example:
Tank 1 ─┐ Tank 2 ─┼─ common balance pipe Tank 3 ─┘
Each tank connects to a larger common pipe.
This arrangement can also include an individual isolation valve for each tank. If one tank needs maintenance, it can be separated while the others remain connected.
Try to avoid unnecessarily complicated pipe runs. Numerous reducers, narrow fittings, sharp turns, and long small-diameter hoses can restrict equalization.
Where Should the Water Outlet Go?
If bottom-connected tanks are properly equalized, you can often take water from one suitable point in the connected system.
You might connect the outlet to:
- A garden hose
- Gravity-fed irrigation
- A transfer pump
- A pressure pump
- Another non-potable water system
The requirements change considerably when a pump is involved.
A pump needs enough available water flow at its inlet. The balance pipe between tanks should not become the restriction that causes the pump to draw one tank down much faster than the others can refill it.
If the pump supplies household plumbing or other pressure-demand applications, pump sizing, pressure controls, backflow protection, plumbing separation, and local requirements become more important. That is usually beyond a simple tank-to-tank plumbing job.
Can Tanks of Different Sizes Be Connected?
Usually, yes, provided the tanks and connection arrangement are suitable.
With a bottom balance pipe, tanks of different widths or capacities can still equalize by water level, not by percentage full.
For example, if a short tank is connected to a much taller tank, the short tank may reach its maximum water level while the taller tank still has unused capacity.
The overflow elevations therefore matter.
Before connecting tanks of different heights, compare:
- Base elevation
- Maximum water level
- Overflow height
- Outlet height
- Tank pressure limits
- Available connection locations
Regarding rain barrel and tank plumbing, compare the storage-vessel configuration for the installation area.
If the shorter tank's overflow is below the desired water level of the taller tank, the system cannot simply be treated as one full-height reservoir.
Connecting IBC Totes
IBC totes are often connected through their lower discharge valves.
This can work well for non-potable storage if the containers are appropriate for the intended use and their previous contents are known.
IBC outlet threads and valve types are not all identical. Do not assume an adapter labeled for an IBC tote will fit every container.
Check the actual valve and thread before buying adapters.
The totes also need substantial, level support. An IBC tote holding around 1,000 liters contains roughly 1,000 kilograms of water, before including the weight of the container and frame.
Avoid stacking filled IBC totes unless the specific containers, supporting structure, foundation, and installation have been designed for that load.
Connecting Rain Barrels
Small rain barrels can use the same basic principles.
A bottom connection lets barrels equalize. An upper connection allows one barrel to overflow into another.
Small connecting hoses are common on barrels, but they can limit flow during strong rainfall. If water enters the first barrel faster than the linking hose can transfer it, that barrel can overflow even though the second barrel has space available.
Keep connecting hoses:
- Free of kinks
- Protected from damage
- Accessible for cleaning
- Properly secured to the fittings
Check them periodically for sediment, algae, insects, or other material that could reduce flow.
Plan the Tank Bases Before Plumbing Them Together
Connecting the pipes is only part of the job.
A large tank places a major load on the ground beneath it. The base must support that load without settling, tilting, or creating concentrated pressure points.
Follow the tank manufacturer's foundation requirements.
This may involve a properly prepared compacted base, concrete pad, or another approved support system depending on the tank.
Do not connect rigid piping between tanks that are likely to settle independently. Movement can place stress on outlets and tank walls.
A suitable flexible section or properly supported piping may be needed where movement is possible.
Protect the System From Freezing
In freezing climates, exposed balance pipes, valves, and small fittings can freeze before the large volume of water inside a tank does.
Frozen water can expand and damage:
- Valves
- Pipe fittings
- Filter housings
- Pumps
- Tank outlets
Do not assume insulation alone will prevent freezing.
A freeze-protection plan may include draining vulnerable sections, locating plumbing below the local frost depth where appropriate, protecting equipment, or taking the system out of service during winter.
The right approach depends heavily on climate and installation design.
What About Water Quality?
Connecting additional tanks increases storage capacity, but it does not make collected rainwater cleaner.
Roof runoff may carry dirt, animal waste, plant material, roofing debris, microorganisms, and other contaminants into storage.
For garden or other non-potable use, useful system components may include:
- Gutter screens
- Leaf guards
- Inlet screening
- A first-flush device where appropriate
- Tank inlet calming arrangements
- Accessible tank cleaning points
A first flush device diverts some of the initial roof runoff away from storage. This can reduce part of the debris carried off a dirty roof, but it is not a complete water-treatment system.
If rainwater will be used for drinking, cooking, or other potable purposes, tank connections are only one part of a much larger design. Suitable collection surfaces, pretreatment, treatment, current laboratory testing, maintenance, and applicable local requirements all matter.
Do not assume that clear-looking rainwater, a single filter, or a UV unit by itself proves that the water is safe to drink.
A Practical Two-Tank Setup
For a typical non-potable rainwater system with two similar tanks, a practical arrangement might include:
- Two tanks installed on stable bases at the same elevation.
- Rainwater entering through screened or otherwise suitable inlets.
- Compatible lower tank fittings.
- An adequately sized balance pipe joining the tanks.
- Accessible isolation valves.
- A suitable outlet for irrigation or pumping.
- A properly sized overflow directed to a safe discharge point.
- Accessible components for inspection and cleaning.
Before filling the system, inspect every fitting and pipe support.
Introduce water gradually and check the tank connections for leaks. Watch how the water levels behave in both tanks. If one tank rises much faster than the other, look for a closed valve, blockage, undersized connecting line, trapped piping arrangement, or elevation difference.
Common Mistakes When Connecting Water Tanks
Using a tiny balance hose
A narrow hose may not transfer water quickly enough during heavy rainfall or high water demand.
Installing tanks at different elevations
Even a moderate height difference can change how the tanks fill and overflow.
Forgetting isolation valves
Without valves, repairing one tank connection may require draining much more of the system.
Reducing large tank outlets unnecessarily
A series of small adapters can create a major flow restriction.
Ignoring the overflow
Every collection system needs a controlled place for excess water to go.
Making fittings inaccessible
Leave enough room to inspect valves, unions, filters, and tank outlets.
Assuming every tank fitting is interchangeable
Thread type, diameter, gasket style, and tank-wall construction vary. Confirm compatibility before making permanent changes.
When to Get Professional Help
A basic pair of rain barrels can often be connected as a DIY project.
Professional help becomes more important when the system involves:
- Large cisterns
- Buried tanks
- Major excavation
- Household plumbing
- Pressurized pumps
- Electrical equipment
- Structural supports
- Drinking-water use
- Cross-connections with a municipal water supply
- Large tanks located near buildings or retaining structures
These systems can involve structural, plumbing, electrical, water-quality, and local regulatory requirements beyond the tank connection itself.
Frequently Asked Questions
Should water tanks be connected at the top or bottom?
Connect them near the bottom when you want the tanks to equalize and behave like one larger reservoir. Connect them through an upper overflow when you want one tank to fill before water moves into the next.
Will two connected water tanks fill evenly?
Bottom-connected tanks on the same level should eventually settle at roughly the same water height. They may not remain perfectly equal while water is entering or leaving quickly, especially if the balance pipe restricts flow.
Can I connect two water tanks with a garden hose?
For small rain barrels, a suitable hose may work. For larger tanks or high inflow rates, a garden hose can be too restrictive. Size the connection for the water that needs to move between the tanks.
Can I connect tanks of different sizes?
Yes, but bottom-connected tanks equalize according to water height. Different tank heights and overflow levels can limit usable capacity, so compare their dimensions and connection elevations first.
Do connected tanks need separate outlets?
Not always. Properly equalized tanks can often feed a common outlet. Individual tank valves are still useful because they let you isolate a tank for maintenance.
Why is one connected tank overflowing while the other is not full?
Possible causes include an undersized balance pipe, blocked pipe, closed valve, different tank elevations, mismatched overflow heights, or a connecting pipe that cannot keep up with incoming water.
Do I need a pump to move water between connected tanks?
Not if the tanks are bottom-connected at similar elevations. Water pressure from the difference in water level moves water through the balance pipe. A pump may be needed for delivering stored water to irrigation or household fixtures when gravity does not provide enough flow or pressure.
Can connected rainwater tanks supply drinking water?
Connecting the tanks does not determine whether the water is potable. Drinking-water use requires a suitable whole-system approach to collection, pretreatment, treatment, laboratory testing, maintenance, and applicable local requirements.
