Volume cannot be analysed alone. When you talk about volume you have to mention what price is doing now (during the volume decline in your case) and what price was doing before (during the volume advance). Only then you may assume supply/demand action behind volume behaviour. SPY and ES volume is not always accurate for the S&P 500 analysis. You may get volume charts for the S&P 500 and other indexes at www.marketvolume.com. See some of their volume based technical analysis at Technical Analysis Volume is quite complicated parameter. The best result could be achieved when you not just follow volume volume indicators but are able to interpret volume action from the supply/demand prospective.