AC - Activated carbon, chemical filtration media
ACRO - Acropora, a type of coral
ATC - Automatic temperature compensating, a feature on some refractometers
ATM - At the moment
BOD - Biological oxygen demand
BTA - Type of anemone (Entacmaea quadricolor) / bubble-tipped, bulb tentacle, bulb tip
CaCl2 - Calcium chloride
CaCO3 - Calcium carbonate
Ca(OH)2 - Calcium hydroxide
CO2- Carbon dioxide
CO3 - Carbonate
CYANO - Cyanobacteria
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
FRAG(s) - Broken or cut pieces of coral which will eventually grow into a new animal
Fuge - Short for refugium
FW - Freshwater
GAC - Granular activated carbon, chemical filtration media
HCO3 - Hydrogen carbonate
HO - HO lighting refers to high output fluorescent lighting.
IR - Infrared, type of light of longer wavelength than visible light
Kalkwasser - German for calcium hydroxide solution or limewater
LED - Light emiting diode
LPS - Large polyped Scleractinian (stoney) coral
LTA - Type of anemone (Macrodactyla doreensis) / Long tentacle anemone
MEQ/L - Milli-equivalents per litre, measure of alkalinity
MH - Metal Halide
MM - Miracle Mud, method of aquarium filtration
ORP - Oxidation reduction potential, the ability to take or give electrons
pH - Measure of the concentration of hydrogen ions, equal to -log[H+]
PH - Powerhead, water pump
PO4 - Phosphate
PPM - Parts per million, equivalent to mg/l (milligrams per litre)
RO - Reverse osmosis, type of water purification
RO/DI - Reverse osmosis, followed by deionization, type of water purification
SCWD - Switching Current Water Director, type of wave machine
SPS - Small polyped Scleractinian (stoney) coral
TFC - Thin film composite, type of RO membrane
Feel free to add addtional ones 