Equipment.
R/O - Reverse Osmosis.
W/C - Water change.
De-chlor - Dechlorinator or tap conditioner.
TDS - Total Disolved Solids.
HOB - hang on Back, (filter)
LFS - Local fish store
QT - Quarantine.
AC - Aqua clear - filter Brand
IPF - Internal Power Filter
FBF - Fluid bed Filter
UGF - Undergravel Filter


Food.
BS - Brine Shrimp
BBS - Baby Brine Shrimp
B/H - Beef Heart.
BW - Black worms/Blood worms.

Discus Strains.
BD - Blue Diamond
RT- Red Turquoise
BT- Blue Turquoise
PB - Pigeon Blood
GP - Golden Pigeon
SS _ Snakeskin
RM - Red Melons
MM - Mellenium Mellon
RP - Red Passiom
MP - Manderin Passion
MG - Mellenium Golden
VR - Violet Refelection
RD - Reflection Deep
SSS - Scorpion Snakeskin
LSS - Leopard Snakeskin
GLSS - Golden Leopard Snakekin
Turq - Turquoise
R.S.G - Red Spotted Green.
SE - Spotted Eruptions
FR - Fire Ruby
FOTF - Flame of the Forest
X - cross

Probably many more but most you can work out by the context its posted in.

Medication/Desease.
Metro - Metronidazole
Prazi - Praziquantel
HITH - Hole in the Head (desease)
PP - Potassium Permangante.
Hex - Heximta


Gerneral Talk.
IME - In my experience
IMHO - In my honest/humble opion
IMO - In my opinion
LOL - Laughing out loud.
HTH - Hope this/that helps.
IIRC - if I remember Correctly
TIA - Thanks in Advance

LPH - Litres Per Hour
LPM - Litres Per Minute
GPH - Gallons per hour
GPM - Gallons per minute

Stores/Sponsors/Supportors
SLS - Slippery little suckers http://www.slipperylittlesuckers.com
AOA - Age Of Aquariums. http://www.aquariumproducts.com.au
G & S - G&S Aquarium http://www.gnsaquarium.com.au
ALA - Aquatic Life Aquarium http://www.aquaticlifeaquariums.com.au
XA - Xtreme Aquariums http://www.xtremeaquariums.com.au
ABF - Australian Betta Forum http://www.australianbettaforums.com/forum/
AFF - AusFishForums (Http://www.ausfishforum.com)
F4U - Fish4u (http://www.fish4u.com.au)