In order to open the Eheim Classic filters, (yours is a 2215) follow these steps.
1. Unplug the electrical cord.
2. Close the Quick Disconnect valves (if you have them).
3. Take the filter to a sink.
4. Open both the Q.D. valves to drain the filter. The water will drain out the bottom fitting ONLY if you allow air to enter through the top fitting.
5. Unlatch the clips that hold the pump head to the canister. Hold the canister body while pulling up the pump head using the handle or tab on the pump head.
You have to allow air into the filter in order to remove the pump head. The only way to do this, is to open the bottom valve AND the top valve to allow the vacuum seal to be broken.