AC - Activated carbon/Aquaclear
AF - Aquarium Forum
AGA - All Glass Aquarium Company
Alk - Alkalinity, measure of buffering capacity of water
ATS - Algae turf scrubber, reef setup technique
BOD - Biological oxygen demand
Ca - Calcium
CaCl2 - Calcium chloride
CaCO3 - Calcium carbonate
Ca(OH)2 - Calcium hydroxide
CC - Crushed Coral (type of substrate)
Cl - Chlorine
CO2 - Carbon dioxide
CO3 - Carbonate
CSL - Custom Sea Life Lighting
CTA - Cellulose triacetate, type of RO membrane
Cu - Copper
Cyano - Cyanobacteria
DC - Direct current
DI - Deionization, type of water purification
DIY - Do it yourself
dKH - Degrees of carbonate hardness, measure of alkalinity
DO - Dissolve oxygen
DOC - Dissolved organic carbon
DSB - Deep sand bed
Fe - Iron
FO - Fish only
FOWLR - Fish only with live rock
FW - Freshwater
GBR - Great Barrier Reef
GPH - Gallons per hour
HCO3 - Hydrogen carbonate
HD - Home Depot
HQI - Mercury (Hg) Quartz Iodide, a type of metal halide lamp
HO - High output fluorescent light
HTH - Hope This Helps
I - Iodine
I2 - Iodide
IO - Instant Ocean salt
IO3 - Iodate
IR - Infrared
Kalk - Kalkwasser, German for calcium hydroxide solution or limewater
KI - Potassium iodide
LFS - Local fish store
LPS - Large polyped Scleractinian (stony) coral
LR - Live rock
LS - Live sand
MACNA - Marine Aquaria Conference of North America, held annually
ME - Marine Environment (type of salt)
MEQ/L - Milli-equivalents per litre, measure of alkalinity
Mg - Magnesium
MH - Metal halide light
MO - Mail order
Na - Sodium
NaCO3 - Sodium carbonate
NaOH - Sodium hydroxide
NH3 - Ammonia
NH4 - Ammonium
NO - Normal output fluorescent light
NO2 - Nitrite
NO3 - Nitrate
NSW - Natural seawater
O2 - Oxygen
ORP - Oxidative redox potential
PA - Premium Aquatics (mail-order vendor)
PC - Power compact fluorescent light
pH - A measure of the acidity or alkalinity of a solution
PH - Powerhead, water pump
PO4 - Phosphate
PPM - Parts per million, equivalent to mg/l (milligrams per litre)
PVC - Poly vinyl chloride, used for piping / plumbing
RC - Reef Crystals (type of salt)
RO - Reverse osmosis, type of water purification
RO/DI - Reverse osmosis, followed by deionization, type of water purification
RR - Reef Ready (refers to aquariums with overflows, etc.)
SD - Southdown Sand
SG - Specific gravity
Si - Silicon
SiO2 - Silicon dioxide
SPS - Small polyped Scleractinian (stony) coral
Sr - Strontium
SSB - Short Sand Bed
SW - Saltwater / seawater
TBS - Tampa Bay Saltwater, Mail Order Company
TFC - Thin film composite, type of RO membrane
TWP - Tap Water Purifier from Aquarium Pharmaceuticals
UGF - Undergravel filter
UV - Ultra violet light
VHO - Very high output fluorescent light

Hope This Helps Everyone

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This should help everyone out! Great post.

Thanks alot Shane!
Allow me to cross link another thread with some other common fishkeeping abbreviations:
Common fishkeeping abbreviations