If you have found midges, flies or wasps around a tank opening, the problem is rarely the tank itself. In most cases, insects are getting in through exposed overflows, vents or poorly sealed pipe entries. Knowing how to stop insects entering water tank systems starts with those access points, because even a small unprotected opening can lead to contamination, odour, blockages and unnecessary cleaning.
For homeowners, landlords and site maintenance teams, this is a preventable issue. The right protection method does not need to restrict water movement or make servicing difficult. It needs to do one job properly - stop pest ingress while allowing the system to operate as intended.
Why insects get into water tanks
Insects are drawn to sheltered, damp spaces. A water tank offers both, particularly during warmer months. Some species are attracted by standing water, while others use openings in pipework and overflows as nesting or resting points. Once inside, they can leave debris, die in the tank, or create a breeding point if conditions allow.
The most common entry routes are open overflow pipes, vent terminations without mesh protection, damaged lids, and loose-fitting connectors. In older installations, the original protection may be missing altogether. In newer systems, the issue is often a mismatch between the fitting and the pipe size, which leaves gaps around the edge.
This matters for more than appearance. If insects are entering a potable or stored water system, hygiene becomes a concern very quickly. In non-potable tanks, infestation can still affect system performance, maintenance costs and general site standards.
How to stop insects entering water tank openings properly
The most effective approach is to treat every external opening as a potential ingress point. That means checking overflows, vents and any outlet or connector that terminates outdoors or into an exposed plant area.
A purpose-built insect screen is usually the best answer. This is not the same as pushing general mesh into a pipe and hoping it stays put. A proper screen is designed to fit the pipe or fitting securely, resist weathering and maintain unrestricted flow. That last point matters. If the screen reduces the effective opening too much, you can create a different problem, particularly on overflow lines that need to discharge at full capacity.
Stainless-steel mesh is generally the preferred material for long-term outdoor use. It offers better durability than light plastic mesh, stands up well to moisture and temperature changes, and provides a more reliable barrier against small insects. The screen also needs a fitting format that matches the application, whether that is BSP male thread, BSP female thread, compression, flanged or a universal connector style.
Where buyers go wrong is assuming one type fits everything. It does not. The right screen depends on the pipe diameter, connection type and how the outlet is terminated.
Protect the overflow first
If you are deciding where to start, begin with the overflow. In many tanks, the overflow is the weakest point because it is open to atmosphere and often left completely unprotected. That makes it an easy route for wasps, flies and crawling insects.
A dedicated overflow insect screen should fit securely to the outlet without reducing the pipe's ability to discharge water. This balance is essential. An overflow exists to protect the tank and surrounding property from overfilling, so any protective component must preserve flow efficiency while excluding pests.
In practical terms, that means choosing a screen made for overflow use rather than adapting a general plumbing part. A well-designed unit will hold its position, resist corrosion and provide a consistent barrier over time. For trade installers and facilities teams, this is also a better maintenance decision because it avoids improvised fixes that loosen, tear or trap debris.
Check the fitting type before you buy
Some overflows terminate with a threaded connection, while others require compression or flange-mounted protection. In retrofit situations, a universal connector can be useful where the pipe end is plain or non-standard. The key is a firm fit with no gaps around the perimeter.
If the screen can shift by hand, it is not secure enough. Even a small gap is enough for insects to bypass the mesh completely.
Do not ignore tank vents and lid seals
Overflow protection is often the main requirement, but it should not be the only check. Vent openings need the same level of attention. A vent without insect screening may not show an obvious problem at first, yet it can still allow pest entry into the tank space.
The vent arrangement also needs to breathe properly. Blocking it off is not a fix. Tanks require airflow for normal operation, so the answer is screened ventilation rather than closure. Again, the best result comes from a fitting that is designed for this use, with mesh fine enough to exclude insects and a housing that remains secure in service.
Lid condition is just as important. Warped covers, cracked inspection hatches and missing seals can all create access points. If a tank lid no longer sits flush, screening the overflow alone will not solve the whole problem. Insects tend to exploit the easiest route available.
What to avoid when stopping insects entering a water tank
Improvised methods are common, especially on domestic properties and older commercial sites. They may appear to work for a short period, but they usually fail under weather exposure or routine system use.
Loose mesh held in place with tape, sealant applied directly over openings, and makeshift caps with drilled holes are all poor substitutes for a proper insect screen. Tape degrades, sealant can restrict access for maintenance, and homemade covers often reduce discharge capacity or leave hidden entry gaps.
There is also a hygiene issue. Materials not intended for long-term use around stored water systems can deteriorate, shed fragments or harbour dirt. If the aim is contamination control, temporary patching is not a dependable answer.
The trade-off between exclusion and flow
This is where specification matters. Finer mesh gives better insect exclusion, but the product still needs to support adequate airflow or overflow performance. On a lightly used vent, that balance is easier to manage. On a high-capacity overflow, the screen design and fitting size become more critical.
That is why a specialist component is usually the right route. It is designed around both conditions rather than just one.
A practical inspection routine
If you are dealing with an existing issue, inspect the system methodically. Start at the tank and work outwards. Check the lid, vent, overflow, any external terminations, and the condition of surrounding pipework. Look for staining, insect activity, nests, damaged seals and any opening that reaches the outside environment.
If there is already contamination inside the tank, the system may need cleaning before fitting new protection. There is little value in screening the openings if the existing debris remains in place. For landlords, maintenance contractors and facilities managers, this is a good point to record what has been changed so future inspections are simpler.
Once screens are fitted, the maintenance burden is generally low. Periodic visual checks are enough in most settings. If the site has heavy vegetation, high insect activity or wind-blown debris, inspect more often. A blocked or damaged screen is unusual with the right product, but it should still be part of routine plant and property checks.
Choosing a long-term fix
If the goal is to stop repeat call-outs and protect water quality properly, choose components that are made for external use and matched to the actual connection type. That means durable materials, a secure installation method and a design that does not compromise the system's intended performance.
For UK properties, that often comes down to selecting the correct overflow insect screen or vent screen in the right size and format. MP Products, through Overflow Systems, focuses specifically on these protection points, which is useful when a general plumbing merchant does not carry purpose-built options.
For a single domestic tank, the fix may be straightforward. For larger buildings or multi-tank sites, there may be several vulnerable points across the system, and consistency matters. Using the correct screened fittings throughout gives a cleaner result, reduces maintenance uncertainty and provides better protection against future ingress.
A water tank should not be left open to whatever happens to be flying past. If you protect the real entry points with the correct screened fittings, you keep the system cleaner, safer and easier to manage over the long term.