Help Setting Up Tank

[waterdrop]

You can cut sponge filters with scissors. The safest recommendation is usually to cut off 1/3 of the old mature foam cylinder and set it in some tank water for the moment. Cut off 1/3 (same size) of the new sponge filter in the other one. Swap these thirds into each respective filter. Now, depending on your preference, you could either run the new filter alongside the old filter in the old tank for a while, or you could put the new filter (that now has 1/3 mature media) into the new tank and begin performing a standard "Add & Wait" Fishless Cycle (per rdd1952's article we all use) and determine whether the "mature media seeding" has either "instantly" cycled the new tank, or has greatly speeded up the fishless cycling process (this is really the safest way, since you can "qualify" your filter by the usual method of seeing if it can process 5ppm of ammonia and the resulting nitrites within 12 hours after the 5ppm addition.) Indeed, it would be very wise to watch it perform this "12-hour" test for a week to be sure you have a fully stable system. (Why not make your new fish feel great in their new home!!)

~~waterdrop~~
ps. I'd think you'd be more likely to preserve "tightness of fit" of the sponge if you cut it like you were sawing off part of a log, rather than lengthwise, but that's without really seeing the situation...

 

[Miss Wiggle]

yes just cut the sponge and swap some of the parts around so that the existing tank has two thirds old sponge and one third new sponge and the new tank has two thirds new sponge and one third old sponge.

Then as WD say's follow the usual process for a fishless cycle (link in my sig if you've not done this before) by adding 5ppm of ammonia each day, when both ammonia and nitrite drop to 0 12 hrs after adding the ammonia then you do a massive water change and add fish the same day (within 24hrs or you'll need to add more ammonia).

The difference will be a normal fishless cycle takes 4-6 weeks, this will be more like 4-6 days although times will fluctuate based on about a zillion variables that we can't measure!