Divorced and permanently separated spouses can deduct maintenance payments to their ex-spouse as special expenses up to 13,805 Euro under income splitting. In return, the recipient must tax the payments as other income.
The income must be declared in Anlage SO in the line "Maintenance payments, insofar as they were deducted by the maintenance payer as special expenses".
Although maintenance payments are taxable, whether you actually have to pay tax on them depends on the amount of maintenance and other income. If you have little or no other income, the tax allowances and rates mean that not much tax will be due.
As a maintenance recipient, you should make your consent to income splitting conditional on the maintenance payer compensating you for any tax and non-tax disadvantages resulting from the taxation of maintenance payments.