Get the largest one your space and budget will allow. The longer between refills the better, IMO. I use 20# tanks on all my aquariums and keep a 5# as a spare to temporarily connect when I have to swap out a tank. Dave _______________________________________________ Aquatic-Plants mailing list Aquatic-Plants at actwin_com http://www.actwin.com/mailman/listinfo.cgi/aquatic-plants