Its often confusing as a beginner with all the abbreviations people use when talking about their marine aquarium, Here we try and unravel some of this jargon for you. Not all of these are fish related some are just good for general use. We have highlighted the most frequently used ones.

AC - Activated carbon
AIO - All In One 
AFAIK - As far as I know
ALK (KH) - Alkalinity
ASAP - As soon as possible
ATO - Auto Top Off
ATS - Algae turf scrubber
BNIB - Brand New In Box
BTW - By the way
BUMP - Bring Up My Post
CA - Calcium
CaCl2 - Calcium chloride
CaCO3 - Calcium carbonate
Ca(OH)2 - .Calcium hydroxide
CO2 - Carbon dioxide
CO3 - Carbonate
Cu - Copper
CUC - Clean Up Crew
CYANO - Cyanobacteria
DI - Deionisation (water cleaning)
DINOS - Dinoflagellates (prest marine plankton)
dKH - Degrees of carbonate hardness, measure of alkalinity
DOC - Dissolved organic carbon or compound
DOM - Dissolved Organic Matter
DSB - Deep Sand Bed
DT - Display Tank
Fe - Iron
FOWLR - Fish Only With Live Rock
FW - Freshwater
GAC - Granular Activated Carbon
GAL - Gallon
GFO - Granular Ferric Oxide (phosphate remover)
GHA - Green Hair Algae
GPH - Gallons per hour
HOB - Hang On Back
I - Iodide
ICH - Whitespot or Ich, Cryptocaryon
IMHO - In my honest opinion  
IMO - In my opinion
KI - Potassium iodide
KISS - Keep it simple stupid
LFS - Local fish store
LPS - Large Polyp Stone Base
LR - Live rock
LT - Litre
Mg - Magnesium
NH3 - Ammonia
NH4 - Ammonium
BNIB - Brand New In Box
NO2 - Nitrite
NO3 - Nitrate
NP - No problem
NSW - Natural seawater
PH - Powerhead
PO4 - Phosphate
RO - Reverse osmosis, type of water purification
RO/DI - Reverse osmosis, followed by deionisation, type of water purification
RTN - Rapid tissue neucrosis
SG - Specific gravity  (how salty your water is)
SPS - Small polyped Stoney Coral
Sr - Strontium
TDS - Total Dissolved Solids
ULNS - Ultra-Low Nutrient System
UV - Ultra violet, type of light of shorter wavelength than visible light
WYSIWYG - What you see is what you get
WS -
Whitespot or Ich, Cryptocaryon

If you think of anymore please let us know