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Moving to a Larger Tank

To be honest, you can read and adapt both setting up a new tank and moving house with a tank to get to the procedure - but in any case, here is how I moved fish when I upgraded to a larger tank.

Before the new tank arrives or is set up
Firstly, I prepared for the filter so there would be a minimal cycle in the new tank. this page on "Beating the cycle" describes in more detail how this worked. However the first stage was to obtain the appropriate filter media for the new, larger filter. As the filter media was incompatible with the filter on the existing tank - and in any case, there was more of it - I put the media loose into a breeding net cage, and put the net cage into the existing tank for a week or two, in order to seed the media ahead of setting up the new tank.

At this stage you might want to order live plants for the new tank. You can keep them temporarily by floating in the existing tank (or even the bath, but the water will cool down and many plants become more brittle when cold, and so will snap when you try to plant them in gravel).

If you are keeping your old tank as well as the new big one
The next thing to do is to begin to set up the new tank. When you will be keeping both the old and new tank active, that's no problem. You can simply begin to set up the new tank, while keeping the old one going. To set up the new tank either follow your usual procedure, or look at this page on setting up your tank in photos and establish it before you introduce fish. It's also good to run the equipment for a few days to make sure they are working well.

A few hours before you want to add the fish, take the filter media that you have been seeding in the old tank, and install it in the new filter. Once the filter is running again and the water is clear, you can add fish to their new home. As with new tanks, it's better to do this in small amounts rather than increasing the fish load all at once, however if you need to move them all at once it should be no problem as long as no new fish are added to the group until the tank is stable again, and the mini-cycle is complete.

Now you have the difficult task of moving the fish. I have two techniques for this. The first is the traditional task of netting and bagging the fish, then acclimatising them to the new tank as if they are new fish. This is probably the most "correct" method. The drawbacks are that when you have netted some fish, the others will be wound up and it's hard to get the last few, even if you use the technique of having two nets and driving the fish into one with the other. They then panic in the bag a little. It's also hard to see in the tank if you stir it up trying to catch them.

The method I use when moving fish from one tank to another nearby is rather different. Because there is no acclimatisation period it only works if the water parameters between the two tanks are closely matched, at least in terms of pH and temperature.

Instead of using two nets and driving the fish into one from the other, I use a small net to drive the fish into a side-on breeding net cage. When the fish is caught, I simply lift the net into the position it would occupy were it hooked onto the side of the tank, i.e. the fish is trapped in the cage, but still in the water. I have waiting an ice-cream container or similar, which was sterilised and then filled with water from the new tank. I've already checked that the breeding net cage fits neatly inside it! Then I lift the net cage up and into the tub full of water, so the fish inside is out of water for less than a second. Then I carry the tub (containing net cage, containing fish) to the new tank. Then I lift the net cage up again and pop it into the new tank - then turn on its side so the fish is free to swim out into the new tank. This technique works rather well for me, particularly with nervous or larger fish, and the fish do seem less traumatised.

After the fish are all over, leave the tank lights off for the rest of the day and don't feed that day, then treat the next day as a "normal" day. Monitor the tank for nitrites and ammonia as the filter adjusts to the load, but you can only expect a small cycle if you've seeded the media appropriately. I found that I needed to add more seeded media after a couple of days (once the ammonia-eating bacteria had grown, when the nitrite-eating bacteria were needed) - but please refer to my page on beating the cycle to get this more accurate.

After a week or so, your tank should be settled and cycled, ready for more fish - which you should add a few at a time, in the usual manner.

Then you can scrub out your old tank, and set it up again (keeping the filter media from that tank in an open container of dechlorinated water so the bacteria stay alive while you strip it down) and then do with the old tank what you will. I spent half a day doing this, and as long as the filter media are kept damp and in a container that isn't sealed (so there is an ample supply of oxygen) - or even popped in a breeding net cage in the new tank temporarily - there is no new cycle in the old tank. I was able to strip, clean, refil and repopulate my old tank the same day with absolutely no problems.

If you are using the new, bigger tank in favour of the older one
In this case, you will have no interim tank, so you will have to follow the procedure for moving house. The difference is that you have already seeded the filter media for the larger tank, so this should not cause you any problems.

Net and bag the fish in the usual way. If you're stripping the tank down and taking it away, you might find it's easier to remove all plants and ornaments (and let the water clear again) so the fish have nowhere to hide and it will be quicker to catch them. You also may want an insulated container such as a polystyrene box in which to store the bags of fish while you install the new tank, so the fish don't cool too much. Your fish shop should be able to save you a box from their deliveries if you ask nicely.

Plants only need to be kept damp; you can keep them in a sandwich bag with a few splashes of water. Ornaments don't need to be damp but should be kept somewhere clean so there is no need to sterilise them before use. Filter media must be kept damp, ideally in oxygenated water (so a sealed container is no good). I keep it in an ice-cream tub of dechlorinated water. Filter media must not go in normal tap water!

Once the fish are out, you need to act as fast as possible in emptying and stripping the tank, removing it and putting the new one in situ. Another thing you can do to prepare for this is, if you're not reusing the gravel, to have washed the new gravel so it's ready for the new tank. Otherwise time taken for the gravel to be cleaned and/or for the tank water to clear can add delays.

Get the new tank in place, full of gravel and water and perhaps a few plants or ornaments. Unless you're going in for extensive planting, you can add them once the fish are back in - it's up to you. Once the temperature is up to speed and the filter (complete with the media you have been seeding) has cleared the water, you can add the fish. Treat them as if they are new from a shop and go through the acclimatisation process. If nothing else, they will have cooled down a little. Leave the lights off for the rest of the day and don't feed them. Then the next day should be treated as a "normal" day.

Keep an eye on the ammonia and nitrites, but you should only go through a mini cycle. Once the tank is stable (in a week or so) you can add more fish - a few at a time, as normal, so the filter can adjust to the new load.

Another thing you can consider doing, if possible, is to use your old filter in the new tank as a supplementary one - at least at first while the tank gets established.

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This page last updated: 01 September 2022

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