Here's a few more.
UGF - Under Gravel Filter
RO - Reverse Osmosis
DI - Deionisation, type of water purification
RO/DI - Reverse Osmosis/Desalination
CTA - Cellulose triacetate, type of RO membrane
TFC - Thin film composite, type of RO membrane
AC - Activated carbon, chemical filtration media / alternating current
DC - Direct current
AGA - All-Glass Aquarium, Aquarium manufacturer
BOD - Biological oxygen demand
Cyano - Cyanobacteria
DIY - Do it yourself
FO - Fish only
FW - Freshwater
SW - Saltwater / Seawater
NSW - Natural seawater
HQI - Mercury (Hg) Quartz Iodide, a type of metal halide lamp
MJ - Maxijet, powerhead manufacturer
Ph - Defines how acidic or alkaline the water is, an extremely important water parameter to maintain.
PH - Powerhead, water pump
PVC - Poly vinyl chloride, used for piping / plumbing
UV - Ultra violet light