I was in the same situation,,I already had house plants, 3 different types of Ivy, a Yucca, and tree palm. I was given a Peace lily from a friend. 1 week later, little gnats flying around. I removed 2 inches of the potting mix from ALL the indoor plants and topped them off with sterile sand, topped with decorative rocks. Gnats can't get thru the sand nor can the larvae. I thought I won the fight, but my containers are the "self watering" type. gnats just flew in where you water from the bottom. Taped all openings and let the plant dry out, so to speak. The best thing is to break the breeding cycle..if the gnats can't get in, no eggs,,,and the sand prevents new ones from getting out. I also used sticky traps around the plants and on the windows..caught around 100+. That's the adults. After about a month, changing traps etc, I'm just about gnat free. 100, 40, 10...So far so good.

The big issue with fungus gnats is the larvae eat the tiny rootlets. I don't know what the trade names in Oz is for B. thuringiensis israelensis, the strain of Bt that works for mosquitos, but it works very well on the larvae of fungus gnats as well. Here, I can buy "mosquito dunks" which are small floating discs that contain the bacteria, and I soak several of them in water for a few days, then use this to water all the plants in the green house. Does a fine job.

Jeff Gillman, hort. prof. at U of MN & frequent contributor to Fine Gardening, recommends staying away from tobacco based insecticides. Not because they don't work against insects but because tobacco has a couple of fatal viruses that infect some of our garden plants, especially geraniums, potatoes, tomatoes, peppers, etc. Roses aren't on this list but because you probably grow some of the above listed plants, it is very easy to contaminate equipment & your own hands with these viruses & transfer them thru out your garden.

Just a guess, but there were probably just a lot more ants being eaten than gnats. Once ants find a food source, they send in the reinforcements. Gnats are not that organized. Also they are probably a bit more substantial for the plant.