Only problem with an 80CF scuba tank (3K to 3.2K) is the number of full fills for an M-Rod. 80CF is going to give something like 5 full-fills. Yeah, it's cheaper than a 4.5K tank, you could tune the M-Rod to work well at lower pressure (say 2.5K) to extend the number of full fills, or you could just live with partial fill ups.