I realized that a lot of us often use acronyms on here, and some people might not know what they mean. Anyone that can add to the list, please do! Looking for just the fishy ones, not the internet ones. *:)

Guess I'll start with a few:

LFS = Local Fish Store
DSB = Deep Sand Bed

WC = Wild Caught (I've seen Water Change also)
MTS = Malaysian Trumpet Snail or Multiple Tank Syndrome

Regarding lights:
PC/CF = Power Compact / Compact Fluorescent
NO = Normal Output
HO = High Output
VHO = Very High Output
MH = Metal Halide

F1 = First generation after wc
F# = F and then a number denoting the generation

UGF = Under Gravel Filter
UGJ = Under Gravel Jets

MTS = Multiple Tank Syndrome

RCS = Red Cherry Shrimp
CRS = Crystal Red Shrimp

ADA = Aqua Design Amano
AS = Aquasoil
PS = Powersand

IMO = In My Opinion

RLC = Rift Lake Cichlids
HOB = Hang On Back (Filter)

GPH = Gallons Per Hour
LPH = Liters Per Hour
WPG = Watts Per Gallon
W/C = Water Change
GH = General Hardness
KH = Carbonate Hardness
No3 = Nitrates
No2 = Nitrites
NH3 = Ammonia
pH = *A measure of the acidity or alkalinity of a solution

Tank dimensions, for example 20H or 20L:
H = High
L = Long
#(20) refers to gallons

when talking about Bettas
PK = plakat (wild type = short tail)
VT = veil tail
DT = Delta Tail
CT = crown tail
DT = double tail
HM = half moon tail
OHM = over half moon tail
BF = butterfly patern
PB = piebald / light colored face
Mask = face same color as body (not black)

bbs = baby brine shrimp

Regarding Water Chemistry / Quality
RO = Reverse Osmosis (sometimes R/O)
TFC = Thin Film Composite (RO unit membrane)
PPM = parts per million
TDS = Total Dissolved Solids (measured in uS, microsiemens)
TWP = Tap Water Purification
S.G. = Specific Gravity
GPD = Gallons Per Day (usually referring to RO output)

Marine
SW = Saltwater (or Seawater)
LR = Live Rock
LS = Live Sand
FOWLR = Fish Only With Live Rock

Other Ones
HITH = Hole in the Head (disease)
QT = Quarantine

CA - (C)entral (A)merican Cichlids
SA - (S)outh (A)merican Cichlids

RTC - Red Tailed Catfish