Any LR you transfer from your 29 to the 20 will have algae spores within it. Your best bet is to find the cause of the algae in the first place. What kind of water are you using? How often are you doing WC? Are you over feeding? How's your lights? Are you running any type of carbon/GFO? Last but not least, what all do you have in your tank? Sorry for all the questions but I'd like to be able to help before you resort to such drastic measures