Forktail blue eyes are a "rainbow fish" (they are 1-2 inches and grouped with rainbows because of the region they are from, but are in their own group) that like very slight brackish conditions. I haven't seen any measurement on the exact salinity. If they are in water with a GH over 8, they likely will do well without any extra salt.
I haven't had Blue eyes, but do have lots of other rainbows and some recommend the addition of salt for them as well. I don't add any. I add the standard dry ferts (KNO3, KSO4, KH2PO4, MgSO4, CaSO4, chelated iron, and CSM+B) which keeps my GH around 13 and they are very happy.
Here are some articles on adding salt to a freshwater tank
I've bredPseudomugil furcatus, forktailed rainbowfish, in soft/tap water no problems. Don't think they need any salt. Now, Marosatherina ladigesi, celebes rainbow, needs slightly alkaline conditions for breeding.