From the FAQ:

Tiny Carp isn't supported
The WoW add-on API makes it too difficult to detect salt water versus fresh water. Therefore, at this time the Tiny Carp companions cannot be supported by Rarity.

Checking wowhead comments, it seems there are some specifics to each of the pets. Can we truly not find a way to detect them? Some ideas:

• Tiny Red Carp: Must be fishing in open water (NOT pool), fishing skill must be X (no failed casts), must loot Golden Carp, must fish in specific area (coordinates/mapID?). Seems doable, but maybe I'm missing something?
• Tiny Blue Carp: TBD
• Tiny Green Carp: TBD
• Tiny White Carp: TBD

The others might be similar?

According to Allara, the saltwater/freshwater thing was what the community decided were the conditions back then. According to wowhead, this isn't correct, though there definitely are conditions (zones where it can be fished from).

We could probably add the subzones similar to Mr. Pinchy? The idea would be to only log attempts while in the areas reported as valid drop zones by wowhead's map (coordinates-based if need be).