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Setting Up a New Filter Without Crashing the Cycle
Upgrading a filter should be a good day. It frequently is not, because the new unit arrives with sterile media, the old one is unplugged the same afternoon, and the tank spends the next three weeks cycling as if it were brand new. Ammonia climbs, fish gasp at the surface, and the upgrade gets blamed for a problem that was entirely avoidable.
The fix is straightforward: never let the bacterial colony leave the system.
Where the bacteria actually live
The nitrifying bacteria that keep a tank safe live on surfaces, and the great majority of them live inside the filter — on the sponge, in the ceramic media, in the biofilm coating the housing. A comparatively small proportion live on the substrate and the glass.
That distribution is why removing a filter is so much more disruptive than a water change. Change half the water and you have removed almost no bacteria. Remove the filter and you have removed most of the colony.
The overlap method
The single most reliable approach, and the one worth planning around:
- Install the new filter alongside the old one. Run both for two to four weeks. The tank has surplus filtration for a while, which harms nothing.
- Seed the new media. Move a piece of mature sponge or a handful of established ceramic media from the old filter into the new one on day one. This is the step that does the work.
- Wait. Two weeks is usually enough for a moderately stocked tank; four is safer for a heavily stocked one.
- Remove the old filter. Take the mature media with it only if you have to; if the new unit has room, leave it in place permanently.
If you genuinely cannot run both — the old unit has failed outright — move every scrap of mature media into the new filter immediately and keep it there. Even a sponge that no longer fits neatly can be cut down and wedged into a chamber.
What to move, and what not to
Move: biological media of any kind, and the coarse sponge. These carry the colony.
Do not bother moving: exhausted carbon, grey floss, anything you were about to throw away. They contribute little and add a load of trapped detritus to a filter you have just cleaned.
Keep the transferred media wet at all times. Nitrifying bacteria start dying within minutes of drying out, so move media in a container of tank water rather than carrying it across the room in your hand.
Watch the first fortnight
Test ammonia and nitrite every day or two for the first two weeks after removing the old filter. What you want to see is a flat line at zero for both.
If ammonia appears:
- Stop feeding for two to three days. Fish are fine without food for that long, and it cuts the ammonia at source.
- Water changes of 25 to 30 per cent daily until the reading returns to zero.
- Do not clean the new filter. It is in the middle of building its colony.
A small nitrite reading following an ammonia spike is the colony catching up and is expected. Persistent readings after two weeks mean not enough mature media came across.
Common ways this goes wrong
Cleaning the new filter too early
People finish the changeover, see debris in the new sponge after a fortnight, and rinse it thoroughly. That removes the young colony they have just spent two weeks growing. Leave the new unit alone for the first month unless flow drops badly.
Adding fish at the same time
A filter changeover and new stock in the same week doubles the load on a colony that is already in transition. Wait a month.
Rinsing the transferred media “so it is clean”
A quick swirl in old tank water to knock off loose debris is fine. A thorough scrub under the tap defeats the entire exercise.
Treating a bacterial bloom as a failure
Milky water a few days after the changeover is usually a harmless heterotrophic bloom feeding on disturbed organics. With ammonia at zero, leave it and it will clear within a week. There is a full guide to telling the different kinds of cloudiness apart elsewhere on this journal.
If you are starting a tank from scratch
There is no colony to preserve, so the question becomes how to build one. A fishless cycle — dosing ammonia to around 2 ppm and waiting for ammonia and nitrite to both read zero within 24 hours — takes three to six weeks and puts nothing at risk. A donated piece of mature media from an established tank cuts that dramatically. Bottled bacteria products vary; they can shorten a cycle but should not be relied on to eliminate it.
Choosing the replacement
If you are upgrading because the tank outgrew the old filter, size the new one for the stocking you have now rather than the stocking you had when you bought the first one. There is a sizing guide elsewhere in this journal, and every unit in our filtration range lists its rated tank volume and the media it takes. Refills for those units are stocked under filter media and cartridges.
Questions about a specific changeover are welcome — tell us what you are running now and what you are moving to. Shipping timings are on the shipping policy page.
From our shelves
Products in this store that relate to what is covered above:
Prices are in US dollars. Delivery timings are on our shipping policy page and the return and refund policy sets out the window if something is not right.
