buy a large bale of media.. or make al famous mix..

and bare root and repot all your plants at once .. isolating the newly potted form the old pots until the job is done ... in fact.. i would do it outside.. all at once.. and then only bring in 'clean pots' ...

potting media is a water management system ... and it gets old.. and wears out.. and when it does.. you can not water your plants properly.. and wait for it ... one side effect will be.. sodden media that wont dry ...

so get rid of it ...

sounds like you are basically killing them anyway.. so why not go for the bare root repotting ...

ken

Agree with @tsugajunkie z5 SE WI ♱ on the Mosquito Bits for larvae, it definitely works. But it takes a while since the adults live on .

To get rid of adults, shield the plant with some paper and blast the soil with a spray of Baygon Mosquito and Fly spray. It kills them fast. No risk of harming bees by using insecticide on indoor plants, so I have no qualms about spraying away.

Agree with above, the BTi (Bacillus thuringiensis subspecies israelensis) will take care of your gnats. No need to ditch your houseplants. It contains spores that produce toxins that specifically target and only affect the larvae of the mosquito, blackfly and fungus gnat - no harm to you, your pets, aquatic and outdoor wildlife if you should use it in outside pots or water features.

It's not an overnight fix though. There really isn't much point in worrying about the adults, they live very short lives. It's the larvae from the eggs they've laid in your pots that you want to address - you want to stop the breeding cycle so no new adults. (I had the worst time with them in moist seed flats one year!) Watering with the BTi solution on your normal watering schedule, plan on at least three weeks to be rid of them.

The products aren't as easy to find now in the ready-to-mix-in-water form not packaged in commercial nursery sizes. There are some Ebay sellers that will sell an ounce or two of the powder at more reasonable prices. Usually listed under BTi, or even Gnatrol.

Or you can make your own BTi 'tea' by soaking mosquito dunks or bits in a container of water, then using that water to water your plants. I'd soak a few hours or overnight.

Be patient and don't give up. You can get rid of these easily with no smell or danger, no harsh insecticide.

Are you sure you’re seeing fungus gnats? could it be whiteflies or something else?

I'd forgotten approx how long an adult gnat lives when I typed my response yesterday. With a little time this morning, I looked it up. Average life span of an adult flying fungus gnat is about 8 days. You are seeing new gnats if they are constantly present - its the breeding cycle that you need to stop and the BTi will take care of that.