Umm, depends on which of the three species you have,adult Anableps anableps is actually more commonly found in fresh water than brackish but usually migrate to brackish to breed, and like many live bearers are prone to fungus without salt. The other 2 species require more salt. The best way that I have seen to keep them healthy is to treat htem like mollies at 1.005 -1.010 sg, after all, why stress them if you don't have to. This means you can you can keep them in very mildly salted water, 1 tablespoon/5 gallons. They also do much better if you keep them fed well with supplemental feedings of mollie or guppy fry, mosquito larvae, and crickets.
As Oddball says though, these like needlefish, are not a good choice to keep with deep bodied predators. they are baitfish.