So with good flow, one can expect it to grow in layer like in jasons picture? A nice layered cap can make a world of difference in the look of any tank.
there r ways to induce the cap to get to be cup shaped. lay the cap flat n position the flow to be just above it n gradually raise the flow or lower the cap.
the idea is allow the cap's growth pattern to chase the flow.
note: the cap also need good random water flow so detritus does not settle inside the cup.
It can be a bit tricky to do, but if you can stand the frag up, it will tend to swirl first.
I set mt frags in small 1X1 slate tiles that come in a sheet at Lowes, etc. With caps I take the tile and run it over the edge of my bench grinder to cut a v-shaped groove. I stand cap frags up in the groove and they've consistantly started to swirl way before they encrust. You could achieve the same thing with a file since slate is very soft.