how do i start a BW tank?

BW is any mixture of seawater and FW. Aquarium salt has nothing, repeat nothing, do do with it. FW + aquarium salt = salted water. FW + marine mix = BW. Marine mix is a very complex mix of materials, far from being undefined aquarium salt - which will not support the life and health of BW and SW critters.

There is no significant difference in SW and FW equipment, except that SW frequently uses more equipment which is not suited to FW. BW tanks rarely use skimmers and such, so it does not apply to most BW tanks. You do need a hydrometer, marine mix, and possibly some different kits - pH is higher, some tests use different chemistry.

So what do you plan on doing with the silver tip? It actually will prefer marine conditions as an adult & grows to 18". A little too large for a 29g.

I am in the same situation as you, I just recently acquired a black fin shark catfish(same thing as a silvertip isn't it?) aa friend of mine bought it without enough forethought, and now that it got sick, it became mine it's still in the "hospital" but doing very well. I however am very excited about setting up a brackish tank.it was sounding very complicated at first, but in actuality I dont' think it's that much more complicated than fresh water really
there is definitely the expense however of upgrading at a later time! But I think these catfish are worth it, they have such beauty and grace!
Let us know what happens!