Double check with your store as to if the fish is in brackish or fresh there. Some good stores do have them in real brackish water, but most won't.
Good job on fishless cycling! What are your readings at right now?
Ask for the actual salinity level. If they just tell you a cup of salt, or 2 teaspoons of aquarium salt per gallon, that is NOT brackish water (wrong kind of salt), and the fish is basically in freshwater. That is easy, once you get the fish you can slowly raise the salinity so the fish and tank get used to it at the same time.
If they tell you something like 1.008, then yes, that is brackish water. If that is the case, you should probably start adding a little marine salt to your tank, but since its still cycling you have to take it slow. I would start adding some salt now, but really, don't worry about it too much, the fish can adapt to slightly different salinities much better than bacteria can.
Basically, either route you choose, you will need a hydrometer (cheap, but can be inaccurate) or a refractometer ($25 or so but very accurate and easy to use), to measure the salinity level and some marine salt (Instant Ocean for example, NOT "aquarium salt").
Then you have two ways of doing it, the slow way, or the slightly faster, but more "technical" way.
The slow way, just add a little more salt with each water change. Lets say every week you do a 50% water change. The first week, add one Tablespoon of marine mix to the replacement water. The second week add 2 Tablespoons of marine mix to the replacement water, the third week, add 3 Tablespoons, and so on. This will raise it very slowly, over a long period of time. Of course, you'll still need to measure the levels with a hydrometer so you know how high you are getting. The difference of a Tablespoon of water should not be enough to push your salinity too high, too fast, unless your tank is way too small. If your tank is at least 10 gallons, and you do 5 gallon water changes weekly, you should be safe. I just did a test.. mixed one Tablespoon of Instant Ocean in a gallon of water, and the salinity was 1.003. So one tablespoon per gallon will only raise .003, considering that .002 per week is the safe level to raise the salinity each week, if you mixed that one Tablespoon into 5 gallons, you will be well under that rate. *disclaimer* mind you, everyone's measurement will be slightly different, due to humidity levels in the salt, the way you fill the measuring device, etc. So make sure you measure YOUR levels with a hydrometer or refractometer and don't go by what worked for me, or what the box suggests. However, one Tablespoon per gallon increase each week should not hurt any tank that is at least 10 gallons. While its not recommended to avoid buying the salinity testing equipment, if you simply can not afford it or find someone to test your water for you, this is the most safe route.
The faster, but more "technical" method, is to raise the salinity .002 each week. Any more than this will kill the bacteria, so its a bit more tricky, you MUST not excede those levels. Basically you need to do a little math.. if your tank is 1.000 (freshwater), you want to raise it to 1.002 your first week. So, you have to figure out how much water to change, and what salinity the new water has to be in order to raise it to where you need it to be. The easiest way is to do 50% water changes each week, with the replacement water being a salinity that is 2x your goal (for the week, not your end goal), minus the current salinity (salt that is already in the water).
(goal salinity of your tank for the week) x 2 - (current salinity, with the 1 before the decimal point ignored for the moment) = (salinity to mix up replacement water) *this only works if you are doing 50% water changes*
Make sense? So if your tank is at 1.000, and you want to raise it to 1.002, you drain 50% of the water, and mix up the replacement water at 1.004 (so 1.002 x .002 = 1.004, 1.004 - .000(current salinity) = 1.004). This should bring the tank to your goal for the week, 1.002. The next week, you are starting at 1.002 and want to raise it to 1.004. Do your math, 1.004 (goal) x 2 = 1.008 - .002 (current salinity) = 1.006. So you do a 50% water change, with the replacement water at 1.006, and your end water will be 1.004. Yes, you will need to be accurate and measure with a device. This method only works for 50% changes, if you do other size water changes I'm leaving it up to you to do your own calculations
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Can someone check my math? I think I did that right?
Okay, so after you decide on a way to mix up the replacement water, there's just a few more things to point out.
Never add the salt directly to the tank, premix it in your replacement water. Most of us prefer to mix our salt and water at least 24 hours ahead of time with a powerhead or airstone in the container, to give it plenty of time to dissolve. Instant Ocean claims this is not necessary, and you can use it immediately after mixing, so its a personal choice and in an emergency, I would not hesitate to do an immediate water change with Instant Ocean.
And the other thing to keep in mind, when a brackish or salt water tank evaporates, the salt does not. So you must make sure you "top off" your tank with freshwater back to the original level, or salt levels will start to rise.
I really hope that helps, I didn't intend to write a book when I started, but I'm actually glad I did. If the math I stated is correct (I'm 99% sure it is, but would hate to tell you the wrong thing), I'm going to work this up as an article so its easier to find the info.
