Matt, from what I have read about starters, bacteria and yeast live in a symbiotic relationship. When first growing a new starter (or in my case reviving one) the bacteria usually can grow at a much faster rate than the yeast and outnumber them 100 to 1. Yeast thrives in a more acidic environment. Adding pineapple, orange, lemon, or lime juice (citric acid) helps lower the pH of the environment an thus slows down the growth of the bacteria giving the yeast a little help.
I have also read of another certain bacteria that tends to take over new starters, but I don't recall the name of it right now. I'll post it if I find it or maybe someone in the know can enlighten us.
I have attempted to make a starter before with just yeast and water and it failed. I've read that without the added citric acid, some 40% of starters will fail. On my 2nd attempt, I added some lemon juice and the starter took off after 7 days. I just carried that same line of thinking over to the new starter I'm reactivating.
I'll see about getting some liter jars at the store later, but I tend to think it's active if there is vigorous activity after 2 hours of feeding. I fed the new starter this morning and it was quite active just after 2 hours. I'll feed it a few more times to see if that makes a difference or not and for good measure.
I'm curious to know how you judge when your starters are active or at their peak performance. How long should it take an active starter to double in volume? Thanks for your continued help.