For actual fungus on a plant, you can spray with 0.2% hydrogen peroxide. Starting with 3% "drugstore peroxide", use 2 ounces per quart of water. That is dilute enough that it shouldn't affect the plant, even if the plant is a delicate seedling.
Here is a home-made insect soapy-spray recipe I saw online and wrote down without the link.
Bear in mind that "soap" is a specific thing, and not all detergents are "soap"s.
1/2 tsp. of dish soap for a quart spray bottle is plenty. Don't think that if some is good, more is better because soap can be harsh if it's too concentrated. Do this at least twice within a week, to get any new bugs whose eggs that hatch after the first treatment.
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After that, to prevent a re-infestation, take your plants to the shower at least once a month and give them a good "rain bath". They will thank you for it. Most indoor plants are more or less tropical in nature, so they like the extra humidity.
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My second try is a soapy water spray. Just a very mild solution of liquid dish soap -
about 1/2tsp. to a quart spray bottle or
2tsp. per gallon if you have a pump sprayer.
A mild dish soap like Method is good.
Detergents with 'grease cutting' ability are a bit harsh.
Be sure to spray tops and bottoms of the leaves, all the stems, everything! If you're spraying in the morning, rinse the plant off with plain water after a short while, like 10min. or so. Soap is a contact killer i.e. you actually need the soap to contact the bug to kill it. You don't want to spray a plant that is in direct sun with a soap solution either. Wait for a cloudy day or when the plant is in the shade.
Again, with the soapy water, you need to repeat the treatment about twice a week for a couple of weeks to be sure of getting the new generations of critters.
Some other people included a little isopropyl alcohol with the soap, say 1/2 tsp per quart.