my mother in law has an old tank, which is currently running with no filter she wants to get a filter the same as ours (an internal 3 stage one with carbon and crystal max) - what i was wondering is my filter has two sponges, if i swapped one of mine for one of the new ones firstly would it affect my tank as my filter is quite happily cycling what we have would the loss of half the bacteria cause us problems? secondly i know it would kick start her filter but her fish are living in pretty high ammonia and hava gotten used to it... would the absence of ammonia and addition of nitrite/nitrate kill them from the shock?