If you are gonna have the 300g done by Sept. you should be fine, although by July would probably be better. It's hard to say "how long?" because growth rates vary. Mine was raised for 6 months in a 55g and 6 months in a 75g. Started at 3.5" and in that years time grew to 14". There is another member here, her silver(raised in a 55g) grew from 6" to 16" in less than 5 months.
The filtration you have on the 55g is fine. I agree with the previous the magnum H.O.B.s are not very good.
The filtration you have on the 55g is fine. I agree with the previous the magnum H.O.B.s are not very good.