Im getting a 15g hex tank and I am thinking about moving my balloon mollies over to it and making the tank brackish for them. I hear they do better in brackish water. How do I achieve this? Right now the 29g has aquarium salt in it, but do I need aquarium salt or salt water salt to make the water brackish? i need some sort of instructions THX

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Tank: 29g:guppies, balloon mollies, swordtail, 2 mollies
Hex: opaline gourami
10g 1F. betta

Here's a quick rundown of everything you will need.

Filter. The best filters for a brackish tank are hang on the back mechanical and bio-wheel filters or canister filters. Undergravel filters will not work properly with a sand substrate. For aquariums larger than 20 gallons, you should use 2 filters on opposite sides of the tank. You will want to have your water turned around 10 times per hour (so you would need a single 300gph filter or two 150gph filters for a 30g tank).
Heater. Never go cheap on a heater. Submersible heaters are much better at spreading heat and are less likely to shatter. Make sure you get a heater marked for fresh AND saltwater use. You also need to make sure the heater is adequate in size. A rule of thumb is 5 watts of heat per gallon.
Glass Top/Light Hood. All tanks should be covered and brackish tanks are no exception. Keep in mind, also, that brackish water evaporates faster than fresh water. If you plan on keeping plants, get atleast 1.5 Watts of light per gallon of water.
Thermometer and Hydrometer. You will need the thermometer to keep the temperature and a hydrometer to measure the salinity of the water. A cheap swing handle hydrometer is fine for this situation.
Substrate. Brackish water areas always are covered in a sandy, silty, or muddy material and I feel that sand is the best option for a brackish water aquarium. You could use marine aragonite, "Minerial Mud", or play sand (which can be found VERY cheap at hardware and home improvement stores). If you plan on keeping live rooted plants, you will need about a 1" layer of substrate (avoid larger layers as they can build up anaerobic spots) and for a fish only tank, a very thin layer is best.
Marine Salt. I prefer Instant Ocean but most brands should work. AVOID Freshwater Aquarium Salt as this is not the proper salt to use in brackish or marine tanks (although it is fine in full freshwater tanks to fight off parasites).
Water Condtioner. As with all aquariums, you will need a product to remove chlorine and chloramine from your tap water before placing it in the tank. Make sure the product is listed for both fresh and saltwater use.

Possible Plants for a brackish tank:
Anubius, Vallisneria, Hairgrass, and Java Fern are best suited to the currents of this tank although plastic is probably the best choice.

Temperature:
80-84 F

pH Level:
7.5-8.5


Salt:
Around 1-2 tablespoons of marine salt per gallon of water or a specific gravity of 1.005-1.010 will be adequate for the full lives of most of these fish. Green Spotted Puffers prefer saltier conditions with around 3-5 tablespoons of marine salt per gallon of water or a specific gravity of 1.010-1.020.

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120 Gal. Reef tank coming this june. This is going to be awesome.

fishteen got it down pretty well. Just be sure to have enough oxygen in the tank, as said, two filters are better than one but you may still need a small powerhead and bubbler to keep things oxygenated well enough. You may also get some mangroves for a plant.

And one last thing make sure the hydrometer you purchase reaches a gravity below 1.005 This will insure you can get proper readings on the salinity of your tank.

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120 Gal. Reef tank coming this june. This is going to be awesome.