Fire ants aren't usually described as tiny. Not that they are a big ant, but not tiny.

I'd get those plants back outside so you can put out a couple of ant baits nearby. I like the Terro products.

Have had a problem in our house with these very tiny ants. Thought they were what folks around here call "sugar ants". Turns out they are another kind, the bug guy called them ferrel ants. These can't be killed by the usual spray because they spread out with several nests around a main colony. Instead they have to be killed with a bait that they then carry back to the main colony. If other measures you try don't work, keep this in mind since it may be the kind of ants you are dealing with. The ants had invaded some of my potted plants also.

Take the plants back outside and sprinkle Amdro around the outside of your pots. Once the ants have discovered it thel will leave the plant and start taking the Andro back to their nest. Take your plants back in after they have vacated them.
Good luck
Hubert
Dalton, Ga.

Check with your extension agent, you need to find where they are coming in as well as using bait. boric acid powder, flour, onion and sugar made into a paste, but you would have to place them where they travel as well as keep it away from other animals like a "roach motel."