Hi
Cool a new and bigger tank!
Can you just clarify when you're planning to 'tear down' the existing tank and 'put up' the new one ?
Would I be right in assumnig that the bigger tank is going to go where the old one was, so you need to house the fish temporarily for a few hours ?
If it's just for a few hours, just use a bucket that has never been used for any sort of cleaning (e.g, no residue of bleach or soap etc), syphon off tank water into the bucket and then net the fish and put them in the bucket. Add a heater and air stone to keep them happy for hours.
When you set up your new tank, if you've got some spare water containers (again must never have had bleach / soap in them), save some of the water from your existing fish tank, so that it's not all tap water (although don't worry too much if you can't do this). Clean the new substrate, put it in the new tank and fill with tap water mixed with old tank water (if you managed to salvage any) to the right temperature and add a water conditioner to dechlorinate the water.
Now the new tank has dechlorinated water in it, add the mature filter and run it to clear the water (don't worry if the water is not perfectly clear before adding the fish, just as long as it doesn't look like soup).
My preferred method of acclimatising the fish to the new tank water is to lower the water in the bucket to about 3" deep and drip water from the new tank into the bucket using air hose to sypon the water, with a clamp on the hose to reduce the flow to a drip. Let the water drip in for about an hour until the water is deeper in the bucket (about 2/3rds full say). Keep a thermometer in the top of the bucket just to make sure the water temperature doesn't go down too much during the process. Now net the fish from the bucket and put them straight in the new tank.
Give them a couple of hours to settle in before feeding them.
Happy moving!
