What species of ant does "black harvester" ant refer to? Novomessor? Veromessor? Some black Pogonomyrmex species?
Generally they need a place to nest (test tubes work by far the best for young colonies), an outworld where they can forage and proper food. Small seeds like Chia seeds, grass seeds, dandelion seeds etc. work best for small colonies. They also need protein-rich food for their brood (dead insects, boiled/scambled eggs, cat food, etc.).
Do you have a queen and do they have brood? Without a queen the colony will slowly die off naturally.
Also losses from hibernation unfortunately aren't uncommon but as long as the queen is healthy they should be able to recover.
Nesting space should only be an issue if there is literally no space to place the brood, in fact giving a young colony a huge nest is a bad thing because they tend to use chambers of it as a garbage dump which can cause all sorts of problems. So a small nest for a small colony should be fine.