Let me second this.
Someone mentioned Rhodia journals, which I have long loved, but I have problems with bleeding when using a flexible nib that puts down a lot of ink. The Rhodia journals are better than the Moleskines, which are still very good with gel pens, but again, they aren't the right journals for fountain pens. I've been through probably 5 of the top brand name journals, but nothing comes close to Tomoe River paper in the Seven Seas journal. And as far as I know, Nanami Paper is the only company out there having them made.
I absolutely love my nanami seven seas writer.
It's got an even more charming appearance with the gfeller kip leather cover which is aging slowly and developing a nice patina.
Nanami has a nice page describing how to deal with show-through here.
For me personally, show-through is not an issue at all. I consider it as background white noise. It's actually a bit pleasing to have your writing show through, it gives your notebook a more written-in feel, instead of pristine, blazing white page. I feel I'd be more likely a victim to writer's block if the page was utterly pristine. A little bit of messy-ness is condusive to my creativity (that's just me though).
Tomoe River being so lightweight and gorgeous, I use an underlying guide-sheet to keep my lines straight. And the translucency of the paper factors significantly in my being able to do that. If the paper is more opaque, then sure I won't have the show-through, but I'll also have a hard time seeing the ruling lines on the guide sheet.
I love how my writing looks on an unlined piece of paper, with no ruling on it, and yet my text is perfectly lined and bordered.
The Seven Seas Writer already comes with ruled paper with very light ruling that is not invasive at all, but I still use a guide sheet to have margins on the sides.