1 to 1.5 lbs./ gallon for rock, correct?The algae will keep your nitrates and phosphates down but your rock will also be a major part of your filtration. I’d say 10-20% of your tank volume. I use a grow light I got on amazon. The light emitted is pink. It is a fairly large light which I have on at night and off during the day. BRS has a really good video on lighting refugiums. It shows several lights from cheap hardware store CFLs to very expensive lights. It shows how well chaeto grows and how well it removes nitrates from the water over time.