Yes it’s tax deductible as long as it’s realized. Unrealized losses are not tax deductible. For example, if you have bought shares of a company and the cost was $100k while the current market value is $85k then your unrealized loss is $15k. If you sell these shares, then you have a negative capital gain or a capital loss as it’s called. You can use this loss to offset it against other capital gains or your taxable income. You can’t offset it if you haven’t sold the stocks but I am almost certain that you can use the loss as long as it has been realized.