You need to decide first whether you want a low-end system (around SG 1.003, up to SG 1.005) or a mid-salinity system (around SG 1.010). The first type is great for gobies, halfbeaks, glassfish, livebearers, killifish, chromides, figure-8 puffers, sleeper gobies and stuff like that. It's very flexible because you can also keep certain salt-tolerant freshwater animals like cichlids, nerites and cherry/amano shrimps. Mid-salinity systems appeal to those people who like migratory, estuarine fish that would, in the wild, be moving between fresh and salt water all their lives. These are things like scats, monos, shark catfish, big puffers, snappers, waspfish and so on. Plants aren't really practical in such tanks.
Give your interest in BBGs and plants, I'd go for a tank around SG 1.003, though set the thing up as freshwater or maybe SG 1.001 to let the plants establish themselves. They will tolerate brackish water much better if they are already settled in. Plants love sand, but it doesn't have much nutrients, so adding a bit of aquatic soil (pond soil) or laterite underneath, mixed with pea gravel, and then topped with sand makes all the difference.
Raising the salinity is easy. There's no "perfect" way to do this, but I'd recommend just doing water changes as per usual, but instead of adding freshwater, add SG 1.001 water (that's water with 3.5 g marine salt mix per litre). Keep doing this for a couple of months. By the end, the tank will be stable at SG 1.001. Over the next month or two, repeat doing SG 1.002 water changes (4.9 g/l), and then the same thing with SG 1.003 (6.2 g/l). This will gently adjust the plants and filter bacteria to brackish water.
Do have a look at my
Brackish FAQ for more information, and try
Brack Calc to understand how salinity works.
Cheers, Neale
