What happened during my germination?

So I started out very successful for the first weekish and the plants looked something like (see below) and they had alittle yellowish leaves near the end of their lives. Started with 6 and have 1 right now. Ph was kept 5.8 - 6.5 it was under a florecent fish tank light just until the 2nd set of leaves were going to pop out. Watered 2 times a day( maby a little much?) but cut back near the end and they still did the same thing. They were in some rock wool cubes. WHAT COULD HAVE HAPPENED???















...Pic used from ARTWIZ

too wet

And only 1 seedling per container plz, thx.