Upgrading your tank: how to migrate your fish over

Why the order matters

Your current tank's filter holds the beneficial bacteria that keep ammonia and nitrite at zero. If you set up a brand-new tank with a brand-new filter and just plop the fish in, you're effectively starting an uncycled tank with a full bioload — a fast route to ammonia spike and stress. The goal is to bring your cycle with you.

Option 1: Use the old filter (easiest)

Set up the new tank (substrate, decor, water, dechlorinate, heat). Move the entire filter from the old tank to the new one and run it there. Move the fish (in a bag or container) and acclimate them to the new water — float and drip if you want to be gentle. Your bacteria come with the filter, so the new tank is effectively cycled from day one. You can add a second filter later and run both for a few weeks if you want to seed the new one, then remove the old filter when the new one is established.

Option 2: Seed the new filter first

If the new tank has a different filter, run both filters on the old tank for 2–4 weeks. The new filter will colonise with bacteria. Then set up the new tank, move the new filter (now seeded) to it, add water (mix old tank water with dechlorinated tap if you like), then move the fish. The old tank can be broken down afterward. This way you don't run an uncycled filter on a full tank.

Moving the fish

Net or siphon fish into a clean container with water from their current tank. Don't feed the day of the move. Float the container in the new tank to match temperature, then gradually add new tank water over 15–20 minutes before releasing them. Test ammonia and nitrite for the next few days; if you see a spike, small water changes and light feeding until it's zero again.

Quick takeaways

• Use the old filter on the new tank, or seed the new filter in the old tank for 2–4 weeks first.
• Never start a bigger tank with a brand-new filter and full stocking — you'll crash the cycle.
• Acclimate fish to the new water; test for a few days after the move.