Some cycles are like that. It's a mystery but you seem to be getting a proper cycle. The first thing I'd add would be a small light in the sump and some cheato. It may sound strange but if you can establish some early it'll help reduce nitrates. You can get a 10w waterproof LED
daylight floodlight off of eBay for about $15 or use a cheap workshop style cone reflector and a CFL daylight from Lowes. It's the least expensive part of the system to upgrade but could save you some salt later on if it's sucking nitrates.