Buy = new ant
Every buy drops a fresh ant into the tank. Big buys drop a whole crowd at once.
Sell = ant leaves
Every sell sends an ant up the tunnels and out of the farm. You'll see it go pale on the way out.
Empty tunnels cave in
When the colony shrinks, tunnels nobody walks through slowly fill back in with dirt.
60% dug = bigger tank
When the ants dig out most of the farm, the tank expands and they get fresh dirt to dig.