Hi, I haven't posted before and am new to this but could someone please help....I have cycled my 60Litre freshwater tank with two platties in the last 6 weeks, all without any probs so far?! I do weekly water changes, fish seem happy, look bright active etc. Now when, if at all, do I change my filter media/cartridge? I have herd numerous answers to this however my thought is if I have spent the last 6 weeks building this fabulous bacteria I do not want to throw 1/3 (1cartridge at a time?) of it away (slim Marina HOB filter holds 3 media cartridges) just as I am about to add another couple of fish ?!?! I know there are many places in my tank these friendly colonies are growing but please advise and thanks for reading my ramblings.