Winter is a good time to lime. Lime increases alkalinity. pH will stay fairly stable at higher alkalinities. Also, alkalinity reduces the toxicity of some chemicals in water. If you have to treat with copper sulfate this Spring or Summer, copper sulfate is less toxic at higher alkalinities.
Treating your pond with salt also is recommended before stocking your pond with a new load of fish. Fish are stressed during harvest and transport. Salt acts to reduce this stress.
If your pond has little inflow and outflow, it may be necessary to break the ice. With a pond that has little inflow, most of the oxygen in water is resulting from contact with the air during winter. If the pond surface is completely frozen for an extended period, this can act as a barrier to replenishing the oxygen in the water. This is especially true if there is snow covering the ice. Most ponds have constantly flowing water so this is usually not a problem.
