To DE or not DE? That is the question.

[PoolDoc]

http://www.unicelfilters.com/pdf/replacement_guide.pdf, page 46:

Mytilus FMY 150 150 7-7/8" 33-3/8" 17-4980 C-7679 4 oz
Note: Product can also be replaced with one C-7699 and one C-7678.

Unicel C-7679 Replacement Filter Cartridge for 150 Gpm Pac-fab @ Amazon

Don't buy, till you've verified cartridge dimensions. You'll get best performance and life if you buy TWO and cycle them periodically. Filter cleaning guide here:
http://www.poolforum.com/pf2/showthread.php?18044

[St8kout]

No, nobody here to told me they used DE. It was the pool guy, both realtors (mine and the seller's) and a few other people. I didn't know any better so I took their word for it. I was going to buy some DE soon too. I did kind of wonder why my filters didn't look like the ones on youtube when I was looking up on how to service them.

I saw that the Unicels are almost half an inch larger in diameter so I opened and looked inside the housing and there's plenty of clearance. I ordered a set from Amazon.

[PoolDoc]

Sounds like you're getting on the right track.