Hoddy --
Bumblebee gobies (BBGs) and mollies make a good combination, as the mollies tend not to eat the bloodworms from the substrate, which the BBGs largely depend on. Conversely, the mollies will eat flake that the BBGs generally ignore. The exact specific gravity (SG) doesn't matter, but I'd recommend about 10-20% seawater, which is SG 1.001 to 1.003.
BBGs don't dig much, so gravel is fine. Most other gobies like to dig, and are happier with a sandy substrate. Anyway, provide the BBGs with large snail shells instead, apple snail shells are ideal, as they seem to like these a good deal.
The
pinned topic covers brackish water aquarium planting very well, but I'd suggest concentrating on Java fern and Anubias to begin with, as these are both very adaptable and reliable plants. They need only a low level of lighting to grow well, and because they are epiphytes (i.e., grow attached to wood, not in the mud) you don't need to worry about the substrate, either. Both are a bit more expensive than the average plant, but they are much better value in the long term because they grow happily under most conditions.
Mollies are fairly big fish, and I'd tend not to keep them in a 12.5 gallon tank. They need at least 20 gallons, and ideally 30 gallons upwards. The average shortfin molly is about 7 cm long, while sailfins are commonly around 10 cm and potentially almost twice that big. Instead, consider guppies or Endlers. If you can find them, "swamp guppies",
Micropoecilia picta, are gorgeous, brackish water livebearers ideally suited to a tank this size.
ICEEGRL -- Good to hear the salt is helping your mollies. Salt helps more if used before the fish get sick, but your mollies should stay healthy now. Popeye is a problem though, and the salt won't help much. It can go away by itself, but takes a while. Bear in mind popeye can be caused by mechanical damage, e.g., the fish scratching itself on something spiky in the tank.
Cheers, Neale
