Gnat Infestation + Toxic Spray Leads to Disaster

This lady I follow on YouTube has this therapy garden in her apartment. It got infested with gnats so, by recommendation of the nursery, she put some 'Horticulture Sand' and some insecticide oil on her plants. She even shot a video of the whole thing:
http://www.youtube.com/watch?v=GrZ3OS_hPn0&feature=related
The oil ended up killing her whole therapy garden:
http://www.youtube.com/watch?v=GrZ3OS_hPn0&feature=related
I'm going to point her in this direction, but I figured some of you smart people might be willing to go over to her and give here some tips and encourage her to try again. I would have recommended using wasps to control the gnats, but having in your apartment leads to it's own problems. Makes dinner time awkward.

Oh my.
Just put a butterwort on your windowsill, pat it on its little behind and of it'll go on a veritable killing spree.
And if its a standard hybrid from the garden centre, it'll be robust and have lovely flowers.

Nematodes safely kill off fungus gnats without damaging plant life. Just water them in, then don't water the garden so much.

I am the therapy gardener UncleRice00 mentioned. Thank you SO much for the kind encouragement & your tips!