I use neem oil, water and a little dish soap. Many of the ready made product use neem oil. If you read the ingredients it often lists 3% or less active ingredient, with neem oil being the active ingredient. I paid $7or $8 for a small spray bottle used up in one application. Then I bought a pint of neem oil for like $16, a bottle of dish soap for $2. 1tblp of each in a gallon of water. That was a couple of months ago. Still haven't used 1/2 the neem.
Make sure you label any spray bottles you put insecticides in so they won't be used for anything else.
The damage done by fungus knats isn't to the green part of the plant. Its done at the roots. They lay their eggs in the medium your using to grow with(coco-coir is one of thier favorites) and the larvae feed on the root hairs and nutreints in the medium. My hydro shop recommended Mosquito Dunks. little round circle of 'stuff' you put in your resevoir. I had to read the package before I would take such a drastic step. When I did I found that the water with the Mosquito dunk in it would still be safe for animals to drink so I tried them. Only need 1/4 of a dunk per resevoir, and they will last thru at least 2 changes. Kills the larvae in the medium.
If you are using soil and not hydro, you could put (sp) diatomacious earth on top of the soil. This stuff cuts thier little bodies as they try to crawl through it. SM90 was recommended by GK as a remedy also.
I use the neem oil, because it controls spider mites as well, along with the mosquito dunks. Hope this helps. VV