wylie72,
Welcome back to Modern!

I'm very new to Ponza (I'm MUCH more of an Infect player), so please take these comments with some healthy skepticism! There are (at least) two Merfolk decks at my second-favorite LGS but I have yet to play either. But I've been reading up/planning for the match-up, so here are some thoughts:

• Keep Beast Within (for killing Lords) and Bonfire of the Damned (for killing everyone) for sure.
• Blood Moon is actually not terrible, since it shuts down Cavern of Souls and Mutavault.
• Keep some mana up when casting Instants and Sorceries if at all possible, to pay the Cursecatcher/Spell Pierce tax.
• Post-board, expect more Tappers (e.g., Tidebinder Mage, Bouncers (e.g., more Vapor Snags or maybe Echoing Truth), and Counters (e.g., Unified Will).
• Some other cards to consider: Firespout, Sudden Shock, and Trinisphere (I run 2x in sideboard and though I don't know yet how it is against Merfolk, it's been great against other fast decks like Elves and Bogles).

Bottom line: If I was boarding your deck against Fish, I'd try ...

• Out: 3x Stone Rain, 1x Blood Moon, 1x Mwonvuli Acid-Moss
• In: 4x Anger of the Gods, 1x Thrun, the Last Troll

Draw well!