I am transferring to my new Fluval Roma 125 today, and the filter is a Tetratec 1000 l/h internal filter. It has 2 compartments with filter media in each, so can I cut up my existing filter media in my old tank and just put it into the new filter and it's ready to go?
If the old tank has been running with fish ,then yes.
Would keep in mind that bacteria on old filter material, will have been in direct proportion to the load of fish in the old tank.
For this reason, I would stock the new tank slowly unless you are moving the fish also from the old tank in which case,you should be ok.
Let's say I move filter material from 20 gallon tank holding five or six fish to larger tank. I then add five or six similar sized fish to the new tank and all is well.
If I were to add twenty fish to the new larger tank, or much larger fish to the new larger tank, then the bacteria from the borrowed filter would not be enough to process the waste(ammonia created by the sudden increase in waste produced by the numerous fish added. Make sense?