From Brotato Wiki
Item
Goldfish is an in-game item that can be bought from a Shop or can be found in a Crate. Most Items modify the Stats of the player, and some of them provide additional effects.
Notes
The Goldfish is a single use item that is used during Shopping, similarly to 
Mirror.
When you reroll the shop while having a goldfish active, the items and weapons on the new reroll will be +1 tier higher than they would've otherwise have been. So if the shop had selected to give you a tier 2 weapon, that becomes tier 3, and it then chooses the specific weapon to give from among tier 3 weapons.
After being used, the Goldfish turns into a 
Resting Goldfish, which does nothing.
The tooltip can be a bit hard to understand. But the goldfish doesn't affect Locked items. And if you reroll while having locked items, you'll see less than 4 items/weapons that have their tier boosted by the goldfish, wasting some of the potential.
- Goldfish can make Tier 4 items and weapons show up in the shop before it would otherwise be possible. Normally Tier 4 can't show up before wave 8+.
- Cursed Goldfish upgrades the next reroll by 2-3 tiers. At +3, this guarantees a full shop of Tier 4 items.
- If you have multiple Goldfish active, for example after using a Mirror to buy two goldfishes, then only one Goldfish is used per reroll. It still only gives +1 to tiers.
- When playing
Curious be warned that since the Goldfish turns into another item after being used, it will show the Green icon even if it isn't the first Goldfish you buy. But you'll end up with 2 Used Goldfishes, which will give you -10% Damage.
Update History
- 1.1.0.0: Added to the game.