I started with 3 seconds 3X/day[Aquapro 75]-that was definitely WAY too much [because nothing was eating anything]; dropped to 1 second at dawn/dusk until some feeding evident, and now at 2 seconds 3X/again. Food is gone in about 20"; for the first 5-10 minutes the BG seem a little intimidated by the HSB and cats, but as the feed disperses they get more involved.
It seems to that your idea of adding mature BG is a great one, especially if you're going to stock six inch LMB this fall. Even if they grow tremendously you'll have a good base in place for next spring/summer.
