How long to up a saltwater tank?

You weren't really clear on one thing, so I'm going to say this just to be safe. In order to convert a tank from fresh to salt, you have to empty everything out and not reuse it (at least for saltwater, excluding equipment). The tank needs to be bleached and dried before saltwater use as well. What I'm trying to say is, you can't just replace the rocks on the bottom of your freshwater tank with sand and add salt to your freshwater tank.

Okay, after you've cleaned your tank and have moved everything, the amount of time you have to wait depends on many this: How much of the stuff from your 55 do you plan on using? Do you plan on taking a ton of rock from your 55, all of the sand, and using everything from your 55 to add to your 16 gallon tank. Do you plan on using like 1 small rock and a small cup of sand, and everything else new rock and sand?

If you go by case 1 and fill your tank with nothing but old sand and old rock, then the amount of time you should wait should be minimal. If you go by case 2 and all rocks and sand will be new, with the exception of a few rocks and a cup of sand, you will have to wait on your tank to cycle.

Either way, I would definately wait and test for ammonia, while feeding the tank for a few days with food to ensure that your tank is in fact already cycled and some of the bacteria moved over nicely. So, basically, feed the tank with food for 1-2 weeks and test the water to ensure it's cycled.

Now, what about your old tank? Are you going to replace the rocks you removed?

Good luck.

Most tanks have to wait a cycling period due to the fact that some of micro organisms contained in the live rock die during the process of transportation and once people get them in the tank and add the water, those dead organisms will soon turn into Ammonia, Nitrate, nitrite and then finally Nitrogen which is expelled from the water in the form of gas. In other words, that's what the process of cycling the tank is, also known as the Nitrogen Cycle.

Another important fact of the cycling period is that several different types of beneficial bacteria need time in order to establish healthy colonies and the bigger the colonies the better natural filtration there will be. However, If your 54 gallon tank is already stablished, it means that the live rock containing in the tank already have large bacteria activity and if all that you wish to just downsize it, make sure that when you move the live rock and the sand, you have them emerged in saltwater the entire time. Have a few buckets with plenty of water in case you have to set them aside for a few minutes till you get everything in place.

It's ok if you keep them out the water for a couple of minutes, but just remember that anything that dies during that process will create a concentration of Ammonia, creating the need of cycling your tank again. I just moved an established 140 gallon tank for one of my clients from one side of the town to another and we kept everything emerged in water, with plenty of water movement. All invertebrates and fish survived and not only that, but seemed like them all adapted very well in a day or two.

In case you have more questions, visit the site below... it has some really good information about cycling a tank, the filtration system, lighting and more.

I usually do not wait at all (replace the water though since you want to start with clean water, not old water). The cycle is in the rock and sand, not the water.

I would of course check the cycle first, before adding fish. Simply set the tank up, test the water. Add ammonia, test again to confirm the cycle