The one year rule
Gains on coins sold, swapped or spent within one year of purchase are taxed at your personal rate of up to 45%, plus solidarity surcharge and church tax where they apply. After one year the gain is tax free. Staking or lending coins does not extend the holding period.
The €1,000 limit
Private sale gains are tax free if your total gains from private sales in a year stay below €1,000. This is an exemption limit, not an allowance: at €1,000 or more, the whole amount is taxable.
Staking, mining and airdrops
Staking and lending rewards are other income, tax free if such income stays below €256 a year in total and taxable in full above it. Mining on a commercial scale is business income. Airdrops are usually taxable when they reward an action by the recipient.
Reporting
Report private sales on Anlage SO of your tax return, using a FIFO or other documented method. The tax office can ask for exchange records, so keep full transaction histories.