12-13-2010, 02:13 AM
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You have an ideal situation here for learning saltwater gradually with the same fish starting out in brackish conditions. Mollys, especially the sailfin mollys do wonderfully in brackish water and can be acclimated to full saltwater if it is done slow enough. Yes, I have done it and I have taught others to do it as well.
One thing to be careful of is population. Working with all the same sex is important if that is the only tank you have available for them, unless you have an outlet that will take the fry when they are very small. Mollys tend to grow almost double in size when kept/raised in brackish water from a young age to adult. The last ones I worked with were silver sailfin mollys and the adults were about 5 - 6 inches long at full grown, and their fry were much larger than the average molly fry as well. 20 gallons would not give you much room for even 1 full spawn on a temporary basis. The good news is that 2 females or 2 males are perfectly compatible together long term, which helps to avoid breeding.
To start this out, rinse everything very well with plain fresh water, set up the tank and get it cycled. Start out as freshwater, introduce the mollys and gradually add salt content with each water change until you reach brackish conditions. There is no limit to the range mollys can tolerate in brackish conditions, so work them up to whatever you are comfortable with and then maintain it there for 1 - 2 months. After 1 - 2 months, slowly increase spg/salinity with each water change until you reach full saltwater conditions. Mollys do real well at 1.023 spg/salinity, which makes them compatible with many of the peaceful small marine fishes such as shrimp gobies, oscellaris clownfish, and many of the shrimp species. The only warning is not to mix mollys with corals as they tend to pick at and/or eat most of them which will damage and kill the coral. If you go this route, to convert to a marine tank and allow room for other tank mates, a larger tank would then be needed.
This is an easy way to learn the ins and outs of saltwater, and if you decide its not for you, its easy to slowly reverse the salinity and take it back down to freshwater conditions in the same manner you worked it up to saltwater to begin with.
Mollys tend to be healthier in brackish water than in either fresh or full saltwater, but healthier in saltwater than in fresh. Many of the common illnesses found today in the standard molly can't survive in a brackish or saltwater environment. The salt also acts as a boost to the immune system of mollys.
Other small brackish water fish that I have always loved were orange chromides and bumble bee gobies, but I think my all time favorite in the brackish fishes is the Indian glass fish (Parambassis ranga). Most often they are sold as freshwater fish but, like the mollys, they thrive best in brackish conditions... (their range of brackish is a bit smaller than that of the molly) If they are purchased as freshwater fish then moving them up to brackish should be gradual like I explained for the mollys.
You have a lot of options open to you. I hope this helps.