Eheim 2017

lots of things change how often they need cleaning. How many fish, what size and how they are fed are somethings that change the answer. Has the tank been running long enough for the bacteria to have a good setup all over the tank or is this a recently started tank? I like to hold off cleaning the first time as the good bacteria seem to be kind of "tender" at first and cleaning can set them back and create an ammonia spike. I go gentle with them until the tank has been running several months.

For normal operation, once past the new stage, I judge when to clean by how much the flow from the filter drops off. If it seems like it is running the same, I often go several months between changing and cleaning the filter. Another way to get a look at what is going on is available with the Eheims. If you shine a bright light on the canister, you can see when it is getting really full of gunk. The Eheims are better than some because they don't just bypass the media when they get dirty so if it is too dirty, the flow will drop off.

I failed to mention how to clean. First make sure it is the right time to clean!

When it is time, I do this:

Unplug
Close both disconnects, open the lines and take the can to a place to work. Leave the tubing on the tank and full of water.
Using old tank water or water without chlorine, I rinse each type of media in a bucket. I squeeze out all the soft stuff but only replace it with new when it begins to fall apart.
Take the cover off the impeller and pull it out. Clean the hole and wipe any grime off the impeller. Carefull with the plastic stuff! Make sure the rubber bumpers are on each end of the shaft and replace it all. A little vaseoline on the big O-ring will make it last longer and go back together easier. Vaseoline is petroleum jelly but it is recommended for the O-rings. Just enough to make them slippery.
When done cleaning, leave the canister with no water, reconnect the tubing and open the valves. If you have your tubing setup right, you may see and hear water run down into the canister and up the other tube to the tank water level. This is likely to blow bubbles and possibly lots of gunk out into the tank. No problems with that. It is just build up from the lines.
Plug it in and it should be ready to go soon. Sometimes there is some air and it gurgles and snorts but soon works normal.

Any way to leave both filters on the tank while the bacteria builds in the newer filter. If not practical to do that, how about moving the bacteria and media from the first filter over into the canister?
What we are thinking about is that bacteria will build up and live on almost all surfaces. Walls substrate, decor and filter media. The idea of most media is to make it really full of the tiny spaces where bacteria can gather. The sponge can have miles of surface compared to a tank wall so much of the bacteria will be in the sponge.

For a new tank, don't clean for a month or so. The 2217 is not likely to show any drop in flow in at least that amount of time so it is safer to just not rock the boat. Testing for ammonia is good at this time when you are in a somewhat "fragile" condition. Like learning to drive a car? Wait a while before going to the freeway!