Nerf

A nerf (symbol: ) is a term that is used to describe the weakening of something, either through updating a game or by releasing a sequel. Nerfs are often done to try balancing towers, by weakening towers that are considered too strong. The opposite of a Nerf is a "Buff," which involves the strengthening of something between sequel games and game updates. By convention, a "nerf" is a balance change in relation to decreasing player ease, rather than reflecting the negative strength of game mechanic "ally" (e.g. towers, heroes, perks) or game mechanic "enemy" (e.g. bloons, map formatting, map obstacles).

The use of nerfs has been introduced since Bloons TD 5 Flash, in which its earlier versions did introduce several series of balance changes for various towers in that game. It was later introduced for all other games of the Bloons TD 5 generation, excluding most console and Steam versions where there has lacked new significant balance changes beyond their older counterparts outside of bug fixes, with exception of Bloons TD Battles Steam. Bloons TD 6 introduces a significant portion of balance changes throughout every single update, and has been a core part of the game for the sake of providing variety in gameplay. Bloons TD Battles Mobile and its Steam counterpart have both still receive balance changes but at a much slower pace.

Definition
By convention, a "nerf" refers to balance changes that make it harder for players attempting to utilize a game mechanic to their advantage. It often decreases "desirable" stats or increases "undesirable" stats, or it could be an overall disadvantageous rework to a specific feature in the game.

Formatting
On Bloons Wiki, a "nerf" is marked with a Red Bloon with a red minus sign, plus following text that describes the nerf.

Simple example:
 * UpgradeName cost increased ($1500 --> $1600)

Refer to the documentation for Template:Nerf for further details.

BTD6

 * Ground Zero, Tsar Bomba, Blade Maelstrom, Super Maelstrom, Tech Terror, and The Anti-Bloon abilities were all stripped of their formerly infinite pierce in Version 24.0.
 * The Tack Zone received a series of nerfs in Version 23.0 and 25.0, nerfing the Path 1 crosspath's pierce and range heavily and removing the popping of Lead and Frozen entirely in compensation for a more defined Path 2 crosspathing benefit and a lower upgrade price.
 * Bloon Impact can no longer pop Black Bloons as of Version 25.0, following Ninja Kiwi's plan to rework damage types in its entirety. Their rationale for this was to balance it out versus Ice Monkey at their respective weaknesses (in this case, White Bloons).
 * Version 25.0 also nerfed Icicle Impale to be unable to hit DDTs without either Metal Freeze or some other Lead-Popping Power buff. Before that, it only needed external Camo Detection or Decamo to freeze them.
 * While Comanche Commander itself was not directly nerfed, the interactions of Alch buffs with subtowers has been removed entirely in Version 12.0, thereby nerfing Comanche Commander's synergy with Alch buffs.
 * After Obyn Greenfoot received a price nerf in Version 11.0, raising his base cost from $650 on Hard to $700 on Hard, he can no longer be afforded at the start of CHIMPS Mode, thereby killing off strategies that relied on an Obyn start. Obyn received even harsher nerfs in Version 18.0, where his Level 2+ "Nature's Wrath" passive buff became restricted to Druids.
 * Berserker Brew and Stronger Stimulant were reworked in Version 6.0 to now also have per-shot mechanics (similarly to Acidic Mixture Dip) and brew reapplication cooldowns. This in turn nerfed them substantially, making them less overpowered on already fast-attacking towers.
 * The Perma-Spike received a series of nerfs in Version 2.0 and later on Version 12.0 due to its sheer power at stockpiling spikes. Before the Version 2.0 nerf, it was so strong that it could tank late-game rounds with little trouble for a very low cost.
 * MOAB Mauler was granted bonus damage to Ceramic Bloons in version 12.0. This bonus has been gradually reduced from +5 damage to no extra damage over several nerfs, from versions 20.0 to 30.0. More specifically, +4 in 20.0, +3 in 23.0, +1 in 25.0, and none in 30.0.
 * Like The Tack Zone, Glaive Lord was nerfed in version 26.0 to no longer pop Lead or Frozen without Red Hot Rangs.
 * Version 27.0 nerfed Monkey Banks so that they could no longer accept Bank Deposits. IMF Loan and above were not affected.

BTD5 Flash to BTD5 Mobile and similar mobile games

 * The much smaller explosions in Pineapples has greatly diminished its already unreliable explosion reliability.
 * Most explosive towers and upgrades, particularly Bomb Tower and Mortar Towers, have smaller explosions than in the Flash versions of the games, decreasing their reliability against defending grouped bloons.
 * The MOAB-SHREDR Spikes and Spiked Mines upgrades for the Spike Factory have a noticeably lower attack speed, hurting their defending capabilities.

BTD Battles Mobile game updates

 * The Bloonchipper was used incredibly often due to it being able to stall M.O.A.B. Class Bloons with its Supa-Vac ability which allowed the user to outlast their opponent, deal lots of damage to them with Super Wide Funnel, and help defend against spaced bloons. While the Bloonchipper did receive nerfs in game updates, they didn't stop it from still being used. However, in update 4.1 the Bloonchipper received a massive nerf in the form of removing the ability for both Super Wide Funnel and Supa-Vac to suck in Z.O.M.Gs. This in turn drastically weakened its ability to outlast opponents and thus lowered its usage from being one of the most used tower in the game, to one of the least used. While the Bloonchipper is nowhere near as good as it used to be, it still performs reasonably well versus spaced bloons, and still has some loadouts that use it (albeit not many), such as Wizard Farm Chipper and Tack Village Chipper. This has also caused a lot of complaints from players since the Super Wide Funnel is much less useful
 * The spamming of M.I.B. Call to Arms to exploit its ability-stacking to immense attack speed and popping power has lead to the popularity of videos about hypersonic towers. However, in Version 4.3.1, the game was updated so that it was no longer possible to stack M.I.B. Call to Arms abilities together. This lead to players complaining about how the M.I.B. Call to Arms is both now still unbalanced and lost a very "cool" exploit.
 * The unexpected Glue Gunner nerf in Version 4.9 where glue no longer duplicates under Monkey Fort has caused criticism over how Ninja Kiwi decides upon balance changes, especially since even stronger towers affected by projectile duplication such as Hydra Rocket Pods hasn't been affected by the nerf yet a much less useful tower such as Glue Gunner gets the nerf.
 * The Spike Factory received a pretty large nerf when the lifetime of its spikes was drastically decreased, which in turn reduced the number of spikes the Spike Factory could produce significantly. Thanks to this the Spike Factory is one of the least used towers in the game since its main use (to catch leaks) isn't very reliable since it can't hold off a lot of bloons before it gets overwhelmed.

Reverted nerfs
Rarely, a nerf may be undone in a later update. Bloons TD 6 has had the following reverted nerfs:
 * Version 12.0 nerfed Perma-Spike to have their spikes last for only three rounds instead of four, among many other nerfs. This specific nerf was reverted in 21.0, so that their spikes last for four rounds again, although this happened coinciding with the Deadly Spikes round lifespan buff. Other Perma-Spike nerfs have not been undone.
 * Version 19.0 nerfed Pat Fusty's Rallying Roar ability (at both level 3 and level 14) to last for a shorter time. The level 3 nerf was reverted in version 21.0 to let earlier levels of Rallying Roar receive a similar amount of uptime to pre-nerf, but the level 14 nerf remained.
 * Version 21.0 nerfed the Mortar Monkey's accuracy without Increased Accuracy, which had unintentionally caused mid-path Mortar to be nerfed too much. Version 22.0 reverted this change but kept the attack speed rework changes.
 * Version 23.0 nerfed MOAB Glue to slow MOAB-Class Bloons by only 25%, instead of the usual 37.5%. Relentless Glue was not affected by the nerf. This was reverted in version 24.0, so MOAB Glue's slowdown is by 37.5% again.
 * In BTD5, Signal Flare could decamo camo Black Bloons and camo Zebra Bloons. This capability disappeared from its BTD6 counterpart until version 27.0, where the capability was restored.
 * Version 19.0 nerfed Arcane Spike to be slightly more expensive in order to deter players from trying to save up for it before Round 40. This nerf was reverted in version 30.0.