The list I posted was the deck I tested. But like I said, there aren't too many fully powered players here. So I started with the null rods in the sideboard and the 2 fire/ice and 1 gorilla shaman maindeck. The majority of decks pack little or no power. They're still mostly very well tuned decks though. There is dragon decks, sligh, food chains goblins, madness, oshawa stompy, r/g beats, something kind of similar to WTF but with bigger creatures, and a couple of partially powered hulk decks as well as a fully power keeper deck or two. There is also a very modern drain slaver deck that a few weeks ago was a stax deck. But he doesn't play here much. There are also a decent number of scrubby decks that usually manage to do decent, elves with land destruction, sui black etc.

I know that this wasn't the ideal meta for fish. But it does cover many of fish's worst match ups. And with the red and green splashes, the deck manages to do very well. The Daze really pull their weight against aggro decks that like to tap out often.

Any reason you prefer the brainstorm Nantuko. It was the worst of all three when I tried it.

I actually tried out sword of fire and ice for a while in a different version of the same deck.

While it was a great card, it was too slow. It eats up 5 mana to draw you a card and deal  essentially 4 more damage a turn. But by the time you can cast it, it's sometimes too late into the game for the card to help you much, especially against aggro.

Mask of Memory essentially lets you dig one card deeper and lets you do this two turns earlier. This is like seeing 4 extra cards by the time you can even cast Swords. Those four cards can usually translate to on average three points of damage (through creatures you drew) any ways as well as a couple more points of damage from lavamancer. Swords only lets you see one card a turn. The difference is huge.

Mask of Memory was a faster, better way to recover from an aggro hoard, for me atleast.

But I will add that I never tried swords for nearly as long as I tried the other three cards mentioned. And I didn't have enough of a chance to truly abuse it.

Can't believe I forgot to mention the null rod issue.

There is a good amount of aggro in my meta, hence the splash for river boa etc.

So I always started with all the null rods in the sideboard with the sideboard fire/ice and shaman maindeck. Almost no one here starts out with null rods maindecked. I've been siding out the masks whenever I sided in the Null Rods and thus never had a problem with the conflict.

That is a very significant point though. If you play null rod maindeck or play in an environment where many people play null rod maindeck, standstill is definately the better option.

I've never found the fact that Mask wasn't blue to be too big of an issue though. There were plenty of other cards to discard and after a turn or two, I got so many cards off the mask that it hardly mattered.

Since more than a few people suggested removing Standstill from Fish for Brainstorm or possibly Mask of Memory, I began testing both cards in Standstill's spot.

I was somewhat surprised by my own conclusions...

Standstill

Pro: Gives a two card advantage for only two mana. When the card works as intended, and it often does, it rivals ancestral recall!

Con: Can only be played when you have control of the board or are at the very least in a better board position. While Fish is designed to achieve this, fast aggro decks can make gaining board advantage next to impossible. Thus this card can sit as dead weight in the player's hand precisely against the decks they need to find answers against the most often. Against every match up from madness and goblins to scrubby beats decks and mirror matches, standstill can be a liability.

Neutral: Some players nullify any card advantage by waiting until you already have 7 cards in hand. But even then you get to keep your best cards and have likely dealt significant damage to them.

Brainstorm

Pro: While the card offers no real card advantage, if you can break a fetchland immediately after casting it, you get three random cards while shuffling away your two worst. Can also be used to hide critical cards from discard effects (not very common any more).

Neutral: The card offers no actual card advantage. Thus while it is useful in a deck packing plenty of bombs, it's nearly as useful in decks that pack few if any real bombs like Gay/r and WTF/r.

Con: If you fail to shuffle your library after you play brainstorm (this happens more often than not even though I specifically adapted my test deck to make room for 7 fetchlands!!), then you get crap cards for the next two turns. So unless the card helps you get out of mana screw or cast a curiosity a turn earlier, it's pretty worthless.

Mask of Memory

Pros:

While more expensive than both curiosity and standstill, it's impact is also more extensive. You get to draw two random cards and dump the worst card in your hand. Assuming that you have a card that's worthless in your current situation, it has the same impact as two curiosities!!

It often gives you cards the same turn you cast it and in a mere two turns usually does more good than standstill. In addition, it doesn't require you to have board advantage to play the card and can thus often get you out of a tight situation where a standstill would be sitting in your hand as dead weight.

Unlike curiosity, it doesn't die when the creature is killed. Thus you needn't hesistate to play it on a soon to sack spiketail hatchling or voidmage prodigy. And don't need to worry about your opponent getting a two for one trade off it.

It also feeds grim lavamancer beautifully and usually enables a  lavamancer activation every turn.

Cons:

It costs three mana to initially cast and equip. Thus you can usually cast it turn three when you can theoretically use curiosity or standstill on turn two. But to be able to get a card off curiosity your second turn, you either need to be playing a playset of Flying Men or you need to make a turn one lavamancer curious. This is something you rarely want to do as lavamancer rarely goes active every turn and when it does, it should be picking off creatures, not hitting the player on the dome. Standstill does give you cards earlier but only if you can get some beats down fast enough, and your opponent fails to. The rest of the time, it sits in your hand as dead weight. And the first time curiosity nets you a card, its not actually card advantage. But the turn you cast Mask, you do get card advantage assuming you have a worthless card sitting in your hand already.

The second con is that the card is indeed very mana hungry. Three mana turn three means you really can't do much else that turn (except for Daze and FoW). But after using the card and seeing how quickly the card lets me accumilate free counters, beats and card advantage. After playing with it first hand and seeing how quickly it turns a hopeless situation into a winnable one, I think the cost is well worth it.

Edit:

In addition, it can't pitch to FoW. But this rarely comes up as there are usually better alternatives to pitch any ways.

Most importantly, Null Rod makes it impossible to equip. In a meta where almost no one including me starts out with their null rods maindecked, this hardly matters, but in most metas, this alone makes standstill the superior choice.

End Edit

In summary...

Standstill is very strong against many archeatypes but a dead weight against many of the ones that fish has the hardest time with (good fast aggro decks). Thus I would only play it over Mask in metas where good fast aggro isn't too common.

Brainstorm, while a very strong card in many decks, just isn't as effective in fish or any of it's variants. It's doesn't provide card advantage and only provides tempo advantage if you are mana screwed or have a shuffle effect. While it works well, both Standstill and Mask beat it out in the end.

Mask of Memory is very underrated. It has so many advantages over the alternatives that I would almost certainly play it over Standstill in any meta with even a hint of fast aggro. And I'm having a blast playing it against control and combo decks as well since Mask will accumilate a massive amount of card advantage in just a few turns. I highly encourage you to test it out yourself. It's more reliable than standstill, helps you a lot more than brainstorm and has a lot of internal synergy with lavamancer. Edit: But as already mentioned, it's lack of synergy with null rod is enough to place it below standstill in terms of playability. End Edit

For reference, this was my test deck...

1 Ancestral Recall
1 Time Walk
4 Curiosity
4 Test Slots (Brainstorm, Standstill, or Mask of Memory)

3 Daze
4 Force of Will
1 Misdirection

3 Null Rod

4 Grim Lavamancer
4 Cloud of Fairies
4 Spiketail Hatchling
4 River Boa

1 Mox Sapphire
1 Island
1 Library of Alexandria
1 Strip Mine
4 Wasteland
3 Mishra's Factory
4 Polluted Delta*
4 Flooded Strand*
2 Volcanic Island*
2 Tropical Island*

Sideboard:
3 Rack and Ruin
2 Oxidize
2 Artifact Mutation
2 Red Elemental Blast
2 Fire/Ice
2 Maze of Ith
1 Null Rod
1 Gorilla Shaman

The mana base varied in each test build.

The 4/4/2/2 configuration listed above is the one I used when I was testing brainstorm. I upped the fetchland count to take more advantage of brainstorm.

In the test deck using Mask of Memory, I initially played a 3/3/3/3 configuration but missed the ability to be able to fetch for exactly the land I needed and thus went back to a 4/4/2/2 configuration.

While testing Standstill, I ran a 3/3/2/2 configuration replacing one of each of two fetchlands with an additional Mishra's Factory and a Fairie Conclave respectively. But I found the fairie conclave to be slowing me down too often (comes into play tapped sucks). Plus it ate up too much mana to activate and I rarely had the mana to do so as there was always something to play with all this card drawing. So I wound up taking out a conclave to get a 4/3/2/2 configuration for the standstill version.

The main deck started as PTW's list.

I replaced his stifle with a daze when I realized that the stifle was too conditional too often while the daze always managed to find a target and was free. The extra daze also proved invaluable against combo matchups.

I also splashed green and made room for 4 river boa when I realized that the boa was easier to cast, was more evasive, shored up for any weaknesses fish had against aggro, a better target for curiosity and was an all around a better creature than Voidmage Prodigy could ever be.

I saw no reason to cut cloud of faries as they always served as a nice evasive body for curiosity and mask of memory and were synergetic with standstill, library and all the decks other two drops. The only situation in which flying men are better is when they are in your opening hand and you have nothing better to do first turn (like drop a lavamancer, or an ancestral, or a sapphire, or a library of alexandria)

I tested call of the herd in place of river boa and found it to be amazing. But I simply couldn't find the room for both and the boa won out in the end. Between the Daze (great against aggro's natural inclination to tap out), Boa and the lavamancer, aggro goes from a difficult match up to a favorable one.

My testing led me to conclude that Mask of Memory is the best card for the slot in any fish variant with standstill only matching it in aggro light metas.