First time grower - not 100% sure what the issue is.

I'm about 20 days into this grow, I lost 2 plants early due to stupidity so this one is going to become my mother. I went aero as it suites my style more than dirt, but I'm having some issues and can't seem to determine exactly what the cause is based on previous threads. It sounds like an MG deficiency but I'm not sure and wanted to see if anyone had an opinion. The issue killed a set of leaves about a week ago and seemed to stop, but today it has reappeared.

Running 4 42w cfls that are approximately 2-3 inches above the leaves.
Room temps are between 75-81
Water temps are between 68-75
PH stays between 5.5-6.5
PPM is currently around 600

Any idea what might be causing these brown spots and ultimately the death of my leaves. The third picture is the first set of leaves that died about a week ago.

IMO man it seems like a ppm of 600 that early may be a little too much. I would try to flush your medium out and restart with a lower dosage of nutes, say 25% and work your way up from there. Hope this helps.