Hello, I have traded for a few years intraday only QQQ and only on ISLD. Now with recent changes I decide to look on ARCA as an alternative since I need to see depth of the market. What I have found is that on QQQ on ARCA there is almost all the time spread about 4-3cents, while on ISLD spreads very often were even 1 cent and were rarely were more than 3 cents . I know that ISLD is more popular than ARCA, but the steadiness with which the spread on ARCA is maintained looks more like a work of a market maker rather than an ECN at which people shooting orders in front of each other. Are there any explanations to that? Thank you.