In Minecraft, Mining Fatigue is a status effect inflicted by Elder Guardians, a hostile underwater boss mob typically found in ocean monuments.
The status effect is considerably inconvenient, slowing players' attack and mining speed, making fighting difficult and breaking blocks more time-consuming.
Fortunately, removing Mining Fatigue is similar to the removal of other undesirable status effects such as Wither, Weakness, or Bad Omen. There is also a helpful mob introduced in Minecraft's Caves & Cliffs update that can remove the effect if players are willing to help it out in battle.
Minecraft: Using milk and Axolotls to remove mining fatigue
Minecraft players heading into an ocean monument may want to bring along a means of removing mining fatigue before they end up in a fight with an Elder Guardian.
In vanilla Minecraft, the two primary sources of removing Mining Fatigue without commands are buckets of milk or Axolotls, who reward players who help them in battle by removing their Mining Fatigue and also providing Regeneration I while in combat.
Since Axolotls are aggressive towards most other aquatic mobs, they have plenty of opportunities to engage in combat, including fighting against Elder Guardians.
In most situations, buckets of milk are easier to come by than Axolotls in Minecraft, so they may be the preferred method to remove Mining Fatigue. Buckets can be crafted from three iron ingots, then used on a cow, goat, or Mooshroom to fill them up with milk.
Killing the Wandering Trader in Minecraft: Java Edition, while it is holding a bucket of milk, can also cause it to drop the milk bucket as loot. Once players are afflicted with Mining Fatigue, simply drinking the milk bucket should remove any status effects present.
Using Axolotls to remove Mining Fatigue can be trickier, as they aren't the most common mobs underwater. However, once players find one, they can attach a lead to it and bring it to the ocean monument where the Elder Guardian resides.
Axolotls are passive to players but are actively hostile to most underwater mobs, and should attack mobs found within an ocean monument. If a player assists the Axolotl in killing a mob that it is fighting, the little water creature will remove Mining Fatigue in thanks.
Axolotls also provide Regeneration I during combat, and this effect is increased for each Axolotl in combat nearby. Bringing these particular water mobs into battle can be trickier than simply using buckets of milk to remove Mining Fatigue, but a bucket of milk doesn't exactly attack hostile mobs on a Minecraft player's behalf.