Milk tanks are used for the Milk Delivery system. 

Small = 25 M - 10 Liters (no level requirement) 

Medium = 100 M  - 30 Liters (level requirement 15)

Large  = 250 M  -  60 Liters  (level requirement 30)

Milk tanks have various stats and sizes. 

Milk tanks can be bought at the main store for Moolah.

Level requirements are based on Farm Hand Level.