how to convert fully cycled freshwater aquarium into brackish and what kind of fish should i get?

Question
Hey Karen, do you remember when i asked you how to convert my fully cycled freshwater tank into a brackish tank, and you told me to replace some of my freshwater with brackish water when I'm doing water changes, well how do i change my substrate, which is currently green sharp gravel, if i changed  it into aragonite or CaribSea Seaflor Special Grade Reef Sand wouldn't i be taking away the good bacteria that was on the gravel,also what kind of sand or substrate should i get(like live sand or dry sand), also what kind of brackish fish would be suitable for my 50 litre, I'm doing the brackish river setup just to let you know, hope your not confused.  

Answer
Hi Luke!
You can change over to marine sand or play sand if you like. You should just try to do it in stages if you can. Like take out 1/4 to 1/3 of the substrate and replace it with sand. (This will be a bit challenging but be patient.) Then give it about 2 weeks for the bacteria to get established on the new substrate and replace another 1/4-1/3 of your substrate. The bacteria will have to get adjusted to the salty water anyway so just be patient and test your water regularly just to see how things are coming along.

Since your aquarium is a bit small. You'll be best off sticking to the smaller brackish species. What I would do first is check around your local petstores and see what brackish species they have in stock. Some good small species to be on the lookout for are- Bumblebee Gobies, Knight Gobies, Orange Chromides,Glass fish (unpainted variety), Black mollies, Celebes halfbeak....

With larger tanks you can keep more species and have more variety of course. ;-) Just make sure to research any species thoroughly ahead of time and never impulse buy.

I really hope this helps!
Best wishes,
Karen~