Moving your fish to a new tank
So, those cute little plecos and bala sharks you bought a year ago are getting a little big for your tank? Come to think of it, all of your fish seemed to have grown since you originally put them in. Now that forty gallon aquarium is looking a little cramped. Not to mention it needs constant water changes because you cannot keep the ammonia levels down. What can you do? In my opinion there are only two options: exchange them for smaller fish or upgrade to a larger tank.
However, I think it is better to upgrade them to a new tank, if possible. I know, bigger tanks are more expensive. They also take up a lot more space. While we are listing their cons let's not forget that they also need new filtration and all in all they will cost more to run than a smaller tank. But, if you have the finds and the will it is perfectly possible to upgrade your cramped fish to a bigger 'pond', so to speak.
Going Back to Kindergarten
If you haven't guessed by now you are going to have to start over. This means new gravel, filtration, tank, and lights. (Unless you just happen to have all of that laying around the house, by chance.) You are, essentially, setting up a system from scratch.
What I would do is get my tank and gravel. Rinse that gravel, or sand if that is your preferred substrate, and line the bottom of the tank. I would then fill the tank with water and add all necessary chemicals. You don't need to have your decorations in place as you can always add them later. I would then set up my filtration and let it run for a few days.
That should all sound rather familiar, since it is how you set up your tank the first time. The only difference now is that you are moving fish from your current tank to the new one. I would try to match all of the water quality conditions as possible to make the transition easier on your fish. I would have the heaters in the two tanks set to the same temperature so that would be one less thing the fish needs to acclimate to, for example. If at all possible I would also try and match pH.
Moving the Fish
Now that you have your tank set up and the water quality is as close as you can get it, it is time to move your fish. I would fill a five gallon bucket 1/4 full with water from the fish' current tank. I would then take a piece of airline tubing and start a siphon from the new tank into the bucket. As soon as the siphon starts tie a knot in the line. The knot will slow the siphon to a slower pace, or even better, a drip.
Theoretically it ail take about an hour to fill the bucket, depending on the speed of your drip. When the bucket is full kill your siphon and use a net to move the fish from the bucket to the tank. It is as easy as that. During the time the fish are acclimating I would add my decorations, if I haven't already done so.
What About Using Stuff From The Old Tank?
Sometimes you don't have the option of saving everything from your old tank. You might need to sell it to pay for your new tank. Or, you might not have the room in your home for two large fish tanks. Either way that is perfectly fine.
You can fill a five gallon bucket and put all of your fish in it while you work. If you absolutely must do this you must work as fast as possible. The first thing I would do is remove the gravel, or sand, from the old tank and put it in the new one. You will , of course have to have a little new substrate as the new tank has a larger surface area than the old one. Once the substrate is in place it is time to move the water. Bucket by bucket I would siphon the water from the old tank and pour it into the new one. Of course, now would also be the perfect time to move the decorations as well. Once you have moved as much of the water as you can it is time to start filling with new water. Make sure that you are adding all the necessary chemicals to the new water!
Once the water is high enough to run your filtration get it running. If you haven't started adding it now is the time to add quick start. Quick start is supposed to cycle a tank instantly. I'm not sure if I completely buy into it, but it is the best you can do in a situation like this. Once the tank is full I would let it run for a little while, this should help to clear the water a little bit. I assure it will be cloudy from all the detritus of the old tank water and the dust from the new substrate. (Yes, there will still be a little dust even if you rinse it before it goes into the tank.)
After the water has cleared a little I would remove some of the water from the five gallon bucket with the fish and then start a siphon to will it with water from the new tank. Once the bucket is full I would then net the fish into the new tank.
A word of warning: this method will be a lot more stressful on the fish and runs a bigger risk of mortalities.
After the Move
Now that you have your new tank running and the fish have been added it is best to watch the fish closely for the next few days. Make sure they are all swimming correctly, aren't gilling heavily, and their coloration hasn't changed. If you encounter any of these issues try and correct them as best as you can. It might mean checking the water quality and doing a water change if need be, or turning the heat up or down, or even adjusting the flow from the air stone.
I would not add any new fish to this system for several weeks. This gives the system the time it needs to find a balance. It also gives the fish time to get used to their new home.
In The End
I hope that whichever method you end up using to move your fish that you are successful. I understand completely that fish often grow larger than expected. Sometimes they grow faster than expected as well. That is fine, that means that your fish are healthy. And that is a good thing. Fish keeping is a great hobby. And a larger fish tank can mean more fish and bigger fish.