They are actually really hard to keep. I never had success, they just slowly died out over time. I put them outside however, and in a year or two they had millions of ants!
But, yes. They are supposed to need a constant changing food type, since they'll stop eating one particular type of food. I tried doing that though andthey still didn't eat anything what I put in there (which was honey, sugar, random food like insects/chicken etc). Eventually they ignored the honey too, so probably got bored of it or something.
The colony I put outside (it got defeated eventually by Argentine ants, was a massive war though)...always climbed up this tree and I saw them bringing down a variety of insects/plant stuff. I saw them always getting sap from the tree too. And, I also very commonly see Tapinoma sessile tending aphids for honeydew 24/7. In fact, every colony I encounter in an urban environment, they are always tending aphid. Which are of course very hard to keep in a formicarium.
Hopefully you have better luck than most, but I recommend just to try a variety of random food. That seems to work best, and maybe try to find sap from a tree. The colony I released liked sap a lot as I always saw them drinking it, which I never tried with the ones I tried keeping. Forgot what type of tree it was, or if that matters too much with sap.
Edited by Vendayn, April 21 2016 - 4:55 PM.