You need to read up on them and see if thats what you really need. How bad can the algae be. What type of water are you using.
Like rob says what brand and size, some brands say a 9 watt will work on a 30 others may say a 15 is needed.
The brand also will tell you what the flow rate should be, some may say 80 GPh
others may say 200 GPH
. So no one can give you an answer with out knowing what brand and wattage your getting. LFS
saying keep it on high sounds like they are not sure what to say. If you run them to fast they will not work.
Find out what kind and do a google search on them and read up before you get one.
Also if your having algae issues them a PB reactor may be what you want to get first.