its weird that you were told that discus prefer to swim vertically rather than horizontally. I would argue the exact opposite. I think the majority of the members here use standard tanks as show tanks. There are many people who use standard 125's I myself prefer standard 75s. Most people choose standard 40 breeders as grow outs when they are young then up to 75s when they are bigger. I guess its ultimately up to you but I strongly suggest you do more research. I do not think that vertical swimming is a discus preference. In the wild they roam in shallow rivers that have a lot more horizontal space than vertical.