A CO2 tank! After 7 years of DIY CO2 and probably hundreds of dollars worth of sugar and yeast, getting a CO2 tank was the one item that on it's own made the biggest difference for me. And to think it only cost me $35 on ebay 
My Eheims are a close second.
Giancarlo Podio
My Eheims are a close second.
Giancarlo Podio