have to agree with qlai here. arbitech I wonder if the strategies you are trying to trade were modeled with unrealistic assumptions about what kind of fills you can get. If most of your alpha is coming from capturing the spread you are playing the HFT's game and that's unlikely to work out well. I'm just starting out learning about intraday but I'm initially looking for strategies that are profitable with market orders (look for the NBBO crossing my limit, not trades), then I can try to improve execution later. For me, saving 0.002 (or even 0.005) cents/share should be icing on the cake, not the difference between profitable and unprofitable.. because I am not HFT.