In an effort to keep down costs and improve water quality issues such as ammonia spikes (our # 1 water quality concern) here are some guidelines that I suggest teachers and students follow if using the Millennium 3000 filter:
Begin in September with 2 new cartridges and run the tank for 2+ weeks prior to adding brook trout eggs to the tank. Add water from a trout stream or use the Bio Zyme additive that comes with your kit when you first purchase the equipment.
Two weeks (roughly) after addition of eggs to the tank, remove one of the two filters, replacing it with another brand new one. Wash that filter out using water from the tank (do this when you are doing a partial water change). Place the rinsed-out filter aside to dry out for future use.
Approx. 2 weeks later, remove the oldest filter medium from your Millennium and replace it with the one that you had rinsed out a couple of weeks prior.
Continue this rotation until you feel that the filter cartridges need to be replaced because you can no longer get them cleaned enough to properly filter the water.
This rotation will accomplish two things. First, it will keep costs of replacement filters down. Second, by only removing one filter at a time from the Millennium, there will always be at least one filter in use with the proper bacteria colonies (proper biology to maintain life) living in it. This will help reduce some of the ammonia spikes we see which cause loss of fish.