I'm new to this brackish fishtank stuff. I have read a lot about it, but is it indeed possible to have a brackish tank with damsel fish?
Certain damselfish, yes. But they aren't the commonly traded ones, so you need to place special orders to obtain them. My FAQ lists at least some of the brackish water species, though there are others I haven't listed, such as Abudefduf sordidus.
Again, at least some batfish are found in brackish water when young, e.g., Platax orbicularis. Whether they're a long term proposition for the brackish water aquarium is debatable, not least of all because they grow so large.
Also, what other types of fish can i make brackish?
Many marine fish enter brackish water, either as juveniles, or for spawning purposes, or simply to breed. Coming up with a list of marine fish that are viable in mid- to high salinity brackish water systems is challenging because there really isn't much literature on the subject. Arothron hispidus, Plotosus lineatus, Acanthopagrus berda and Lutjanus apodus are among the "big" marines-in-brackish species, but a better bet for the average system would be Microcanthus strigatus, a fish that looks (and behaves) a lot like a scat. Some of the Dendrochirus spp. have been reported from brackish water, and again, these are small enough to be sensible choices.
When keeping marine fish in brackish system you do need to provide marine-quality filtration. Most species will need fairly high salinity to do well, around SG 1.012 at minimum in most cases. Whilst Arothron hispidus for example will survive for weeks, months even, in freshwater, there's no point keeping them that way.
Cheers, Neale
