If you’re noticing tiny black or gray flies hovering around your sinks, drains, or even near your actual septic tank, you're not alone. One of the complaints septic system owners vocalize is the presence of septic flies. This often lead to the next question:
“How do I get rid of septic flies?”
Septic flies are also known as drain flies, sewer gnats, or moth flies. These annoying pests will often breed in moist, organic matter, which makes a septic system and plumbing lines a preferred breeding ground for them to thrive in. While septic flies are not inherently dangerous, they can be persistent, unsanitary, and very annoying.
Let’s explore how septic flies form, why they may keep coming back, and how to eliminate them once and for all, both safely and effectively.
Septic flies are very tiny insects (1.5-5mm in size) with fuzzy bodies and wings. You’ll most often see them:
Around floor drains, sinks, or showers
Near the septic tank lid or vent pipe
Around basement sump pumps or leaky plumbing
Hovering in warm, humid areas (bathrooms, laundry rooms, kitchens)
Septic flies will breed in gelatinous film, sludge, or organic waste buildup, which makes septic lines, traps, and scum layers the ideal breeding ground habitat.
At Wholesale Septic Supply, we’ve seen the following common culprits with septic fly issues:
Rarely used drains (guest bathrooms, floor drains)
Clogged or slow-moving drains
Organic buildup in pipes or tank
Faulty septic venting or broken seals around lids
Cracked pipes or standing water near the drain field
If you happen to be noticing flies inside your home and around your septic tank, it's time to take proper action.
Identify the Source
Start with checking:
Sinks, floor drains, tubs, and basement drains
Around the toilet base or under sinks
Near your septic tank lid, riser, or vent pipe
Any area with standing water or mildew
You may end up discovering adult flies, or a jelly-like film where the septic fly larvae are developing.
Always use a septic-safe enzyme drain cleaner to remove any biofilm and organic buildup. Avoid using bleach or any other form of caustic chemicals, as they’ll kill the septic bacteria and could damage your pipes.
Pour the cleaner into each affected drain at night
Let it sit overnight (don’t run water)
Repeat for several days for full effectiveness
Running water through rarely used drains at least once per week to prevent stagnant water buildup is a good idea. You may also choose to plug these drains with stoppers when not in use.
If flies are beginning to congregate near your tank:
Inspect the lid and riser for cracks or gaps
Make sure your septic tank lid is airtight
Clean off any organic debris, residue, or algae
Septic flies may end up breeding even in smaller pools of stagnant water. Look out for the following:
Soggy soil or pooling near the tank
Leaky outdoor hoses or downspouts
Cracked irrigation or sprinkler pipes nearby
Redirect any excess water and fix any drainage issues as soon as possible.
To more effectively catch adult flies while treating the breeding ground:
Place UV or sticky fly traps near drains and septic areas
Clean the trap and replace every few days
These septic fly traps will assist in reducing the septic fly population while your enzyme treatment works behind the scenes.
Don’t use bleach or chemical drain openers, as these kill beneficial septic bacteria
Don’t pour boiling water into pipes — it can warp PVC and doesn't solve the root problem
Don’t ignore the issue, as septic flies will reproduce quickly
At Wholesale Septic Supply, we always advocate for safe, long-term solutions over temporary quick fixes.
Once your flies are gone, keep them from coming back:
Even simply flushing unused toilets once per week may help in preventing fly-friendly buildup.
Pump your septic tank every 3-5 years, inspect for cracks, and keep the lid sealed tightly.
What do septic flies look like?
Septic flies are quite tiny (1.5-5mm in size), have gray or black bodies, with fuzzy wings. They’ll often resemble miniature moths and will choose to rest on walls near drains.
Do septic flies mean my system is failing?
No, not necessarily. However, they do signal organic buildup, poor ventilation, or stagnant water which could be a sign of other issues.
Will bleach kill septic flies?
Yes, bleach will likely be able to kill septic flies, but it will also kill off the good bacteria in your septic tank as well. Bleach is not septic-safe, and it may end up creating more problems over time with repeated use.
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