You don't "need" the random contaminants from your local river/drinking water; nitrates, phosphates and silicates aren't going to do anything other than cause algae issues, regardless of whether the tank is FW or SW. Phosphates and nitrates, in particular, are the primary constituents of plant fertilizer (ever seen the four-quadrant NPK logo with numbers? two of those quadrants describe nitrate and phosphate levels, typically much higher than K). No river source is going to be steady in these levels because of environmental conditions, and on any given day can have 100 times the amount seen on the average day, without warning. That doesn't even consider any industrial sewage that could be present in the water on a given day (your nearest Walmart that carries fertilizer, will wreak havoc on your tank if there's a storm), issues with acid rain, or straight up chemicals like horomones, antibacterial agents, and various pharmaceuticals like the ones recently found to be causing mutations of wildlife in the Potomac River (D.C.) and seemingly-pristine areas (at least relative to D.C.) like Wisconsin. You need something in the water, which should be added later. Kent, in particular, makes a Discus conditioner and a Cichild conditioner. Tetra makes one for "blackwater" fish like discus and tetras. Seachem actually makes one specific to Tanganyika cichilds, and another for Victora/Malawi cichilds. Anyway, the point is, there's nothing in tap water necessary for even the most demanding FW tank types that isn't available elsewhere, and tap water largely carries the same risk whether you're running SW FO/FOWLR or FW.