What single thing...
 

... has made a huge difference for you in the hobby?

For me, plumbing my RO into my top off. Life is so much simpler.

Honorable mention goes to my water change hose and refractometers :)

Biopellets! Only way I could have sps dominant and 22 fish:) in a 90g

~Tony

Internet info and forumns!!, hands down giant hobby changer.....good and bad lol :)

LEDs. ;)

That is a good one. No such thing when I started, we only had usenet forums at the time. alt.aquaria was it -lol

Auto top-off, simple, effective, priceless.

Would probably have ended up spending wayyyy too much money listening to lfs if not for the forums!

one thing
 

books-knowledge is king

my controller
 

My wireless aquarium controller and my web cam. I can watch over my tank from pretty much everywhere, so it's peace of mind.

This^^^^. After you finally figure out who the people are that actually know what they're talking about.