fish tank filter question?

I had a Fluval 405, which is good up to 100 gal. It took a dump after a few years, and I bought a Fluval 305, which is good up to 70 gal. My fish tank is only 50 gal. or so, and the canister to the 305 had a hole in the bottom of it (which I found out AFTER running it for an hour or so) The store didn't have anymore 305's left, but the motor fits perfectly on the canister for the 405.

All right, my question is, the 405 had 4 trays for media, whereas the 305 has 3. Will it affect anything to use the 305 motor with the 405 canister, and still only use 3 trays, or does the size of the canister affect the operation. Basically, I would have a little extra space inside the canister.

Thanks in advance
 
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