Never seen ants IN the toilet before, but a couple years back, my TT did pick up an Argentine ant infestation while out in California/Nevada/Arizona on a trip. (Argentine ants are especially small black ants, and not native to North America.)
After trying ant baits and sprays, I got rid of them by sprinkling about 8 ounces of food grade diatomaceous earth in the "basement" where it was generally dusty anyway and in the area under the furnace and where the plumbing runs. DE works only when kept dry, as these areas are, and where it is normally kinda dusty/dirty and not kept spotless by normal housekeeping. You might try it.
And while we're at it, I bought 10 lbs of DE (since the shipping was more than the stuff itself) for use around the rancho, and I probably have 8 lbs of it left, in case anybody'd like some. S&H is your dime.