run the two together for 2 weeks to allow the beneficial bacteria in the new fluval to gather,
you Can run the two together if you want all the time but if you don't want to allow at least two weeks of running the two together

A good idea would be to put in the Fluval, get it all ready to go, and then remove the undergravel filter. That way, when all the poop and waste that is trapped under the gravel is released, the new canister can easily pick it up. Also, it would be a VERY good idea to put a handful of gravel (that is currently in the tank) in the new filter so the bacteria can colonize the new filter media.

conect the two in series such that the fluval will drive the undergravel filter either as regular flow or reverse flow. that way, you speed up the maturing of the nitrifying bacteria in the fluval. after a while you can disconnect the undergravel in case you dont want to run it anymore. or you can run the two of them in tandem permanently

I love UGF's. It seems that everyone is going away from them but I use them in my 2 big tanks and have had no problems. They are so much less expensive to run and keep the gravel relatively clean without a lot of vacuuming.

I had one tank with an UGF and an external filter for awhile. The power went out and when it came back on the impeller on the external didn't go and the whole thing overheated nearly causing a fire.

I only use an external now on my 55 gallon goldfish tank and would love to get an UGF for it if they made one to fit my tank.

If you really want to switch over I would run both filter for awhile then just disconnect the UGF and leave it in place. Taking it out will disturb your whole tank and dump a lot of waste into the tank.

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The only reason to remove your undergravel filter is if you have digging fish. There is no other reason to remove it. under gravel filters work very well for what they were intended to do...which is strickly to grow massive amounts of nitrifying bacteria...they should always be used with a good quality power/ or canister filter. My vote is if you have a community tank, leave them both running....if you have african cichlids/large SA cichlids...you might as well remove the undergravel as digging eliminates their efficiency altogether.

This filtration system worked for many decades before newer filters with bio beds came out...and they work now!!

Just run them both.

You have the gravel in the tank anyway, so continue to use it as biofiltration. Just add the new filter, it will cycle in it's own good time without affecting the tank in any way, and 2 filters is better than one :-)

Ian