Im getting two Top Fin 60's for my 125 and I have a 30 on it too. Should I put water from the 55 gallon into the filters to grow bacteria? Or should I fill the tank up 3/4 way with hose water then add the 1/4 of 55 tank water?
Water from the tank will provide a more mature system which is beneficial to help the bacteria grow, but isn't sufficient for seeding... If the water is dirty then it won't be any good.
Best thing is old filter media, since that is where the concentration of bacteria will be.